Here is a game played between a junior and adult. I don't want to mention names so I will refer to the junior as A.
It is a good idea to play through games once they have finished and try and see where you went wrong. There are no hard and fast rules as the best way to do this, but I like to try and verbalise my plans. Often its obvious why that plan failed you can look back a few moves and see if anything could have been done to change the outcome. Even better is to get a stronger player to look at the game afterwards.
Adult player V A.
1. d4 d5 Black takes a share of the centre and opens lines for his pieces.
2. c4 Nc6 Develops a piece towards the centre, although this blocks Black's best break move for a beginner its still a good move, but as the player progresses he will need to understand that 2...e6 or 2...Nf6 are better. The pawn on d5 can be taken and may result in loss of time.
3. Nc3 Nf6 Another piece joins the fight. So far so good.
4. f3 e5 Fights for the centre, but perhaps is too aggressive.
5. dxe5 Nxe5 Breaks up White's centre and gets better piece play.
6. Nxd5 Nxd5 Looks like a good idea, but this loses a pawn. Black could have played Nxc4
7. cxd5 c6 A clever move to fight for the centre, But Bb4+ and 0-0 is just getting developed and may be better for Black
8.e4 Bb4+ 9. Bd2 Bxd2+ 10. Qxd2 O-O Black finds the suggested moves anyway and although a pawn down has decent chances.
cxd5 I want Black to make another move instead here, but I can't suggest a better move. It's just that White's king is in the centre and he hasn't developed his king side pieces at all yet. You would think that Black could start an attack from this position. cxd5 is ok, perhaps Qf6.
12. Qxd5 Qxd5 13. Rxd5 f6 Any pawn moves in front of the king have to be well and truly justified! If another move for example Nc6 can be played in stead it should be preferred, as the weakening of the king side is often an invitation for tactics.
14. f4 Nc6 The knight ends up on c6 anyway.
15. Bc4 Be6 White threatens a discovery by moving his rook. Black does well to block that trick with his last.
16. Rc5 Rac8 - Black loses with this move. After Bxc4 the game is still well and truly on. When the game gets complicated like this. You have to try and see all the threats. First look at any checks and captures, if Black had done this he should have spotted the 5...Bxc4 was ok and that 5...Rca6 6.Bxe6+ was a disaster. This is a material mistake and is common in junior chess, in fact Black had played very well up to here, probably better than is older opponent. Black quickly collapses after this.
17. Bxe6+ Kh8 18. Bxc8 Nd4 19. Bxb7 Rb8 20. Rc8+