I made a lot of bad moves in this game, but exactly why are they bad?

Please have a look at the third chapter in this study. I was playing black.
Can someone of you please explain what is bad about moves 7, 12 (is not even shown as a blunder, but it is bad because I lose my advantage), 24 (I thought trapping the bishop was worth a try, it wasn't, but why), 28, 29, 30, 37, 49.
Move 15 was also bad, but for that move at least I have an indication why it was bad (I put my queen and rook on the same diagonal, where they could be attacked both by the white bishop)
Also, some general recommendations to avoid these mistakes would be nice.
Thanks for helping out.

In moves 7, 12, and 24 you could gain material (free pawns!) but after you didn't the evaluation goes down.
In move 24 you completely expose your king by trading the two pawns on f+g 7 for the bishop (see all the checks afterwards, hard time for any counterplay).
The alternate moves 24...bxc4 25.bxc4 Bxc would have led to a better coordination of your pieces (=active light square bishop!) in addition to a gained pawn. As a beginner, that's what I think about these moves.

Thanks for your answer, I'll look into it.
You're probably right about the free pawns.

Scrolled thru your game real quick now. Opening is passive, if you play the Colle System (pawns d4, e3) then your bishop should go to a more aggressive d3 square. Not to e2. Allowing doubled isolated h pawns when you castled on the kingside is not a good idea, weakens your king. You should look ahead and quickly check what you and your opponent can capture at every move. Then you began to hang pawns, leaving them undefended, get Susan Polgar’s World Champion’s Guide to Chess, and do all those puzzles in that book many times over until total automatism, this will help you raise your rating a lot. Those are super easy tactics, many of them, and all very practical and typical of normal games.