General

soft wheels for rough surfaces (city, streets), hard wheels for smooth surfaces (boardwalk, smooth roads)
large wheels for longer lasting pushes, small wheels for fast acceleration (if you go big, make sure the wheels don't hit your deck while carving)
get a nice pair of steel bearings, they'll last longer + withstand whatever muck you ride over. make sure you clean 'em if they get super dirty, or they'll slow you down (gunk usually gets caught between the parts and, well, ..)

speaking in broad strokes, but that's essentially all you really need to know as a beginner. weight, style and terrain can affect your purchases but unless you're doing suttin' crazy, you should be good. also peep longboarding forums - there are tons of helpful guides and members who'll (probably) have much more thorough and meaningful advice than basilers

you could also probably just walk in to your local skate shop and ask whoever's working, they'll 95% know what's up and help you out

I just started skateboarding 2 weeks ago and already injured myself.

As a newbie to the whole thing, and the fact that skateboard is kind of different from longboarding, I probably don't have any amazing advice, but maybe what I say could help you prevent the same mistakes that I made.

Anyways, stretch your heels, I pulled a tendon on my heel by not doing this. My heel is already a bit messed up, but I ended up pulling my heel by pushing to hard. I partially blame my wheels for it. There are no actual skate parks where I live, I have to skate on the streets, which has cracks, rough terrain, other impurities, etc. I currently use 53mm 95a wheels, which you wouldn't be using since they're skateboard specific.

Wheel sizes are measured in mm of the diameter of the wheels. As a longbaorder, you'll probably be using 60mm or larger wheels. The hardness of wheels come, as far as I know, ranges from like 70a - 101a, higher numbers means the harder a wheel. Softer a wheel is, more mobile it is, can get through terrain, and won't have as much impact/shock when you go over impurities on the ground. Since you're long boarding, you're probably gonna be cruising more, so go on the softer scale. Harder wheels are meant more for tricks and whatnot. Though soft wheels are better for terrain and stuff, hard wheels help with powerslides, which can help for more abrupt stops. Smaller wheel sizes are also better for tricks and what not, but larger wheels can get over more impurities in terrain. Longboards generally have larger wheels.

Careful not to get wheels that are too big. If you lean to turn, your deck might hit the wheels, so find a size that's right. Also, when you start off, keep your trucks kind of loose. Since you need your own trucks, you'll probably have to buy new ones, which are a pain to break in. If you keep them tight, especially when you haven't broken into them, you won't be able to turn easily, and would have to add extra force and pressure to turn.

Hardware is mainly preference, though I think something around 60mm~64mm and 75a~90a would be fine for you. Since I mainly just cruise around, I plan on replacing my wheels with something around 58mm / 85a. And remember to stretch! Also, flat shoes will help with better grip.

u dont maintain a longboard they will break
i suggest u buy the best wheels trucks and bearing since those last forever
enjoi is a good brand