It seems like the loss of that D pawn was a mistake, and a pretty big one. In the few things I have learned, it is white's ultimate duty to gain space, and black's duty to try and exchange peices. Seems like the other way around. When you have a blocked center like this, you have two options. play on the wings, or bash open the center. in this case you seem to have been very passive, and didnt gain much space. Your best idea would to be smach open the middle, in my opinion, and try to pick an opening that suits the game, and not just a standard one. Though you developed, your opening didnt grab as much space as his pawns. Just my thoughts.
looking for a plan

Without being in a state to calculate properly, 8.Nxb5 jumps out at me. To give some ideas (don't take them as gospel, just throwing out moves):
The general idea should be to open lines, possibly by repositioning your pieces and a pawn push. Even 6. Nxe5 may be worth looking at.

In the original post, you start wasting turns as of about move 9. Get the rooks on d and e files, and just rip the middle open. Eventually you'll have a really, really ugly sequence of discovered checks (from your rooks) against his centralized and unprotected king.
Just keep getting pieces out and the combination will take care of itself eventually.

Some great ideas here, thanks! I would never even have considered saccing a piece without an immediate way of winning it back.
@remellion: I didn't play 7.Bc4 becasue he is very likely to follow up with b5 then. Is the idea 8.Bb3?

Yeah, 8. Bb3 is one safe idea, keeping the bishop on a good diagonal.
However I see violence! 7. Bc4 b5 8. Nxe5 dxe5 (8...bxc4?? 9. Qh5+ and mate on f7) 9. Qh5+ Kd7 10. 0-0-0+ (since the bishop isn't on d3) and now you have options like checking on f7-e6 or even saccing again on b5.
Basically, since his king is in the center and he has no pieces developed, you need to open the game up, even if that means sacrificing a piece.

When you have a big lead in development (you have a lot more pieces out into the battle, not pawns, but your back row pieces out) look for a way to open the position up. Like if you have 4 out to their 1 for example. Opening the position means getting rid of some pawns to get at his king. If that requires trading a piece for a pawn or 2 then think hard about the best way to do it. Development is important, use it. Imagine if you know you're going to get into a fight after school & you also know your opponent could have as many as 7 friends to maybe help beat you up. Well, don't show up with only 2 friends. You be the 1 that shows up with more friends. It's better to be over prepared (get your pieces out, then use them) than under prepared. In cases like this it's better to be firstest with the mostest (that's an old chess saying that normally applies to mutual attacks). It could apply to attacking first.
Hi,
After move 8 in this game, I have developed most of my pieces, while my opponent has been pushing his pawns. I'm a beginner at chess, but I suppose that I would be doing well here. However, I have no idea how to continue now. I can't wait for my opponent to get all his pieces into play, but I can't see much to attack either. Especially since the movement of my pieces is restricted. I tried moving some pawns myself, to at least stop him from advancing some more in the next moves, but it seems a bit weak.
Any advice on how to continue?
(and which awful blunders did I already make?)
Thanks!