About two months ago I had to replace my rear derailer, and ever since then I haven't been able to shift my gears properly, I've tried taking them off and redoing it, adjusting the two screws for the gears, and buying a new cable. But nothing I've tried works. So please if anyone can help I'd greatly appreciate it. I'm not sure if it matters but I have a 2012 diamond back wild wood

What derailleur did you install ? Is it a compatible model ?
What are the current symptoms ?

You can read this for possible help: https://www.parktool.com/blog/repair-...nts-derailleur

The basics are fairly straightforward and the same for almost all derailleurs.

1- start with a square hanger, which puts the RD parallel with the wheel. You can check by shifting to any gear where the cage is fairly vertical, then look from behind the bike and compare it to a straightedge placed against the wheel.
2- the 2 limit screws don't affect shifting, they only determine the extreme ends (limits) of the travel range.
3- the B-screw on the back of the upper body sets the derailleur height, which should be so the upper pulley is close but not touching any sprockets. check on the larger ones and set the gap at about 1/4-3/8"
4- the actual shifting is controlled by the cable, and adjusted with the cable (trim) adjuster.

Those are the basics. I suggest you find a tutorial and follow the instructions to the letter, which should leave you in good shape.

First question is why did you replace the RD? The circumstances might help identify the root cause.
Second is the RD a direct part# replacement or a different model / part# ? What did you have and with is the new one?
Third question is describe "not shifting properly." By trying to describe a consistent and repeatable symptom you may actually discover the problem. (Bonus points!)

After all of that, adjusting a RD is a methodical approach, no magic, did you follow a process from the Park Tool website or other step-by-step instructions? Other potential problems are bent hanger and less likely chain issues.

Go back to your bike, put it in a stand or flip it over, and step through the shifting then describe what you see.

My guess is you have a wonky derailleur hanger. If your derailleur isn't aligned to the same plane as your rear wheel, it'll never index properly.

based on this part of the post, I suspect the real issue is that the OP doesn't understand how derailleurs work, and how to set up and adjust.

This is why I listed some key points, then sent him to review tutorials.

Plenty of good suggestions so IamDannyphantom can get back to us with more solid information.