what do you consider trustworthy resources to consult on trans topics?

depends on which topic you're interested in :)

for anything HRT-related, transfemscience.org is a fantastic resource.

for clothing/fashion-y stuff, i found this blog pretty useful as kind of a starting point to get an idea of what i should even be thinking about when looking for clothes and how to put together an outfit, but honestly nikki's advice of just trying shit out is probably the only way to really get a feel for it. wandering through thrift stores and trying on anything that catches your eye is maybe something worth trying out.

other than that, i'm not really aware of much in terms of general "trans information" sources. reddit is (or used to be) a pretty good resource (e.g. i found answers to a lot of my early questions on r/MtF or r/asktransgender), but since the entire platform is rapidly going down the shitter i wouldn't bet too hard on that. for the most part you'll probably get better answers to any non-trivial question by actually asking people yourself, especially folks in your local community - they can do things in-person that we internet strangers cannot (e.g. accompanying you to the store), and might come with extra bonus information about people nearby (e.g. "oh, you're looking for tucking underwear? i know sb who doesn't need hers anymore, maybe she'll be willing to pass it on").

affirming undergarments

since you appear to be in the US, i'm gonna ditto nikki and recommend En Femme, out of all the brands of tucking panties i've tried they seem to be the best

where else would you recommend I look for online community and any suggestions on finding IRL community?

by far the most useful online communities will be ones with close ties to your local area. there's probably some sort of regional discord or signal group or something for your area, which more likely than not is spread by word-of-mouth. (for example, a baby trans probably wouldn't be able to show up to more than a few gatherings anywhere in the central/northeast switzerland area without getting invited to join the [NAME REDACTED] discord). other than that, if you're on matrix then #gv-apply:chat.blahaj.zone might be a good place to start (more info).

as for finding IRL community:

• check out the websites of higher-level trans/queer organizations and advocacy groups for your broader area (like country/state level, bigger is better cause it'll be easier to find. you're looking for anything with an event calendar (example), or better yet, a list of lower-level organizations (at city/county level) so that you can find groups/meetups/events/whatever nearby (you're looking for something like this).
• if you're studying, your uni likely has some kind of queer student organization (although i guess YMMV depending on political climate)
• try looking into other feminist activist groups, they might have something for you! again an example from my area: probably the most significant recurring event for transfem people within 100km of where i live is technically run by a local branch of the national feminist strike collective, which has the unfortunate consequence of making it impossible to find via any "conventional" queer organization. that said you'll probably have more luck finding this sort of thing through word-of-mouth once you've already got "boots on the ground" in some other queer space.

EDIT: that said, depending on where you are, you may have to resign yourself to having to travel rather far out of your way to find anything IRL. e.g. the area i live in is populated almost entirely by retired farmers, finance bros, and expat families who never leave their bubble, so i end up having to travel 45 minutes to the next major city in order to actually find any events.

EDIT2: added note about tucking undies