Groups are always going to be the hardest part.
Ask your friends/coworkers if they want to join you. Personally knowing people goes a long way. Even if you guys are just acquaintances, it's better than joining a group of strangers. Generally 5 is an ideal number (counting yourself), with 3 being a "minimum" for a good game. You can get away with 2 if an adventure is specifically designed for it, but generally the best experiences will be when you have 5-6 people total.
My last campaign I asked a group of co-workers if they wanted to play, and we wound up with a group of 4 that played every 2 weeks. I'm also in a campaign with my fiance's friend where it's 5 of us every week (my fiance was invited and she asked if I could come along, even though I didn't know the organizer originally). There's a third campaign where my stepdad asked me if I wanted to join him; we're a group of 7 meeting every 2 weeks.
But you still totally can join a group of strangers, if you want! As others have suggested, start by asking your local comic book/games shop. Places that sell Warhammer figurines are also good spots to start; there's a lot of overlap in the communities and typically they'll have DnD nerds too.
Once you have a group, the next hardest part is figuring out who will be the Dungeon Master. The Dungeon Master is the referee; they call the shots, decide what happens, and act as "the bad guy". The Dungeon Master is a player, too; they just have different responsibilities than the other players. It's a lot of work to be a DM, but it's very rewarding.
My first-ever campaign I became the Dungeon Master, because nobody else wanted to do it and I really wanted to play DnD. I didn't have a firm grasp of the rules, but I tried my best and worked with my players as much as I could. You want to make sure that they're having fun, and you want to facilitate communication as much as possible. Players will have lots of questions - "is it okay if I do XYZ?" "What gods should I worship?" etc. It's the DM's job to handle this sort of stuff and make sure everyone is on the same page.
This also applies to other things, as well. Typically the first session with a group is "Session 0", where everyone tests that everything is working and people are put on the same page. It's not expected that people play in session 0; the goal is to establish boundaries.
What themes are going to be in this campaign? Does the campaign allow guns, or is it strictly fantasy weaponry? What level do players start at? Are there any homebrew rules? How are player stats generated? Are people okay with descriptions of slavery? Sexual assault? Is it okay for players to romance other players? Where do people draw the line? You make these decisions in session 0 so everyone is onboard and comfortable - make sure everyone is heard and everyone has collectively agreed on where that line is.
A great example - a player I had in a campaign had arachnophobia, so I reskinned my spiders into something else for her (without telling her they were really spiders - I described them as giant rats).
Certain campaigns may have other rules. For example, some campaigns may exist in a world where magic is outlawed or rare - this means that spellcasters aren't common, and being able to do magic is an oddity or a crime. Other times there may be restrictions like "this takes place in a tropical jungle, so if you wear heavy armor your character will always be hot and may have issues."
It's up to the DM what rules exist in the world (or not). Even if it's an established setting/world, they may put restrictions on things they don't want to deal with. Some DMs will allow "oddball" races like Aarakocra (Jarnathan from the DnD movie is an Aarakocra). Other DMs ban them because they can fly, and flying creatures break puzzles - or they don't think a race like Warforged is a fit for their setting. Some DMs really care about your backstory so they can use it against you later; others are sticking to a script where you are a blank slate and your backstory doesn't matter.
If you suggest the campaign, it's very likely that you will wind up as the DM, unless you can con someone else into it. Some groups even rotate the DM, where 3-4 people serve as the DM with their own campaigns and each week it's a different person running things.
Only once the DM has been sorted and you've had your session 0 can you really think about character creation. Your DM can even help you come up with characters during session 0, if you'd like (that's part of the reason why session 0 exists).
Every campaign will be different. The rules on what kinds of characters will be good fits will always change. Some DMs are very combat-heavy; others will go weeks without a combat encounter and rely heavily on roleplaying and intrigue. You can't really come up with a "best" character because you will always have strengths and weaknesses; you might be good at combat but bad at puzzles.
Either way, you should strive to have a balanced party. Generally you want a "Striker" (max single-target DPS), a "Scout" (stealth), a "Face" (high Charisma, good at deception), a "Blaster" (max AoE), a "Controller" (control where people can or can't go), a "Defender" (tank, draw aggro, protect your squishies), a "Healer" (take a wild guess), a "Librarian" (high knowledge/investigation), a "Support" (apply buffs), and a "Utility" (out of combat magic).
Of course... you don't have that many characters! So you pick and choose which roles your party is lacking, doubling or tripling up on some roles. For example, the Healer is frequently dropped entirely because combat healing isn't very good in DnD. The Blaster is often also the Librarian or Face, the Striker can be the Scout, the Controller can be Utility, etc. You need to look at what everyone wants to play and figure out where the party is lacking, then create a character to fit that gap.
The "standard" party is generally "Cleric, Fighter, Rogue, Wizard". Each character is usually covering multiple roles (although classes will handle some roles better than others). Note that in Baldur's Gate you run into all those exact classes within 5 minutes after the tutorial... almost like the devs did that on purpose!
You don't have to run that exact party, but it's a good guideline for how you should build a party. Druids can substitute for Clerics or Wizards, Barbarians or Paladins can substitute for Fighters, Monks or Rangers can substitute for Rogues, Sorcerers/Warlocks can substitute for Wizards, heck even Warlock/Wizard can substitute for Fighter with the right build.
A lot of builds you see online will be "minmaxed" builds. They'll do things like multiclassing and bending the mechanics of the game until they break. Your DM may not allow this. Get a vibe for what's okay or not before just grabbing builds online just because they seem powerful.
A helpful resource is DnD Beyond. They have guided character creation and interactive character sheets. Game mechanics are turned into easy buttons and stuff is calculated automatically instead of making you do math.
If your campaign is on DnD Beyond and someone has bought books through them, they can opt to share those books with everyone in that campaign. This lets you have access to content you'd usually need to buy just because your friend has it - very handy! IRL you'd just share the book, but DnD Beyond makes it very easy.
I hope that helps!
DnD is a lot of fun, but the challenge is always in keeping the group going. It's very easy to miss a session and have the whole thing fall apart. Make sure that standards are set and that attendance is expected (with exceptions, of course). Most campaigns will never be completed; they die to schedule conflicts after a year or so. That'll be your eternal enemy, so do your part in trying to fight it.