I think that was a good lesson.  

I'm a little surprised such a strong player would choose this opening in correspondence style chess. It's really not a bad move, but it should be considered inferior to a more common opening such as 1. e4

Provocation. But the thing is quite playable: it doesn't compromise White's game significantly.

Well, it must be somehow brilliant because it puts a knight on the wings. Knights on the rim are dim, as the saying goes. It's an opening that's named for a reason.

1.Na3? what an opening!

This move is OK, but as White is being provocative, there is something to be said for being provoked: 1...e5, say, staking out territory in the centre.

I would play for the centre e.g. 1...e5 or 1...d5 or 1...c5.

Black can already consider himself better, and as long as he doesn't get cocky, he should come out with an advantage.

To avoid any misunderstanding I must say that we both belong to the same club. There's a forum in our club where lower rated members have the chance to play against a 2000+ rated member with certain rules (it's a way to improve your level). One of those rules is that, as a handicap for the higher rated player, the lower rated player always plays with the white pieces. That wasn't really a game from that forum but I think he tried two things with these moves: makes me feel relaxed (as a said at the beginning it was my first time against a 2000+ player) and give me the chance to take the initiative of the game. In fact, this game was included in that forum afterwards (commenting the game is another rule of that forum the lower rated player has to accomplish).

Again: provocative. But for all the cost in time, White is presenting no weaknesses (with the possible exception of the slightly vulnerable f2-square). I recall a spoof article about this in which the 'perfect game' was described. It opened in very much this sort of fashion, and continued that way: knights emerging from behind the pawns and submerging again... nothing being given away... Draw!

This move can not possibly be bad...

Actually, it was a good move. Although it was the wrong diagonal, the bishop is still in a good place. Black is still well developed.

I prefer either 3...c5 or 3...Nc6 preparing e5

Let me guess (I haven't looked any further)... g4 will come very soon.

This was a worthless move in this position.

I'd say White was definitely telegraphing g2-g4. As Black's light-square bishop (LSB) has already committed itself to f5, it seems hardly worthwhile to prevent the bishop's move to g4 now. In any case, it is often better to play the pawn move AFTER the bishop gets to g4, force Black to decide what then to do with the importunate prelate, and gain a little time thereby.

Of course the other meaning is pawn storm from kingside starting with h3, g4 etc..

This is where the opening books and primers can be a bit misleading. White is indeed playing 'bad' - but not exactly 'badly'. He knows what he is doing. He is telegraphing his aggressive intentions towards the King-side, and is willingly taking on the risk of compromising his game thereby. The move is actually quite reasonable in the situation that has developed. Surprisingly enough, 5.Nh4 was also a perfectly reasonable move here. Black has the edge, yes, but it not a big edge - not a big edge at all.

Next time probably make sure you have a concrete variation or definite compensation before going after a "tactical shot"

If Black's edge had been worth a pawn, or the pawn's equivalent, this would have been reasonable. I've tried the same sort of gig against easy19 myself, in much more promising circumstances, and reaped the same reward meted out to Black in this game. The idea is always worth a look, but usually it is best left alone. 5...Bg6 was good and safe.

I would have risked pinning the knight against the rook at e4. But your right, Bg6 is a safe move for the Bishop.

5...Nxg4? why sacrificing the knight without need. Against such a strong opponent, you are lost now.

I would consider white only slightly better here. (Although it's always hard for the other side to prove it).

White is the equivalent of a pawn ahead, and this early in the game, a knight is worth possibly a fraction more than 3 pawns. If Black had gained some intangible in addition to the pawn for the knight - time, or space - then the disparity would be less than it is. Black has a slight edge in space for the nonce, but that wont last...

(!) This looks surprising at first, as Black is given the option of wrecking White's K-side by 7...Bxf3 8.exf3 c5. But White would have a lot of open lines to play with, and the pair of bishops to use them. White is figuring that it is high time to stake his own claim in the centre - and the doubled f-pawns have something to contribure there.

On the other hand, if you do nothing you're gonna lose anyway, so why not try to attack and make the best out of your previous mistake?

'Passed pawns should be pushed.' OK, but not yet maybe. 'Get yer pieces out' is the more pressing imperative. So I would suggest any of ...Nc6, ...Nd7, ...Bd6. I think I prefer the middle one, covering e5, but also leaving the c-pawn free to advance to c5. At that, is 7...c5 at once worth a look... ? No. 7...c5? 8.Ne5 Bf5 9.e4! Bxe4 (...dxe4 meets precisely the same response from white  10.Bb5ch! Black would have been in deep strife. The position indicates a development of one of the unmoved minor pieces.

Note that had Black developed the knight, or the bishop to d6, this knight sortie would scarcely have been playable.

Taking advantage of black's mistake.

(?) Although this is a mistake, it does indicate a mind flexible enough to look beyond the obvious. The refutation, though obvious at first sight, has certain subtleties that White had to take into account...

Terrible mistake that loses the rook. Best but still losing was 10...Rxh5

The lesson here is this: when things are starting to go turnip shaped is not the time to try playing quickly. Even after 11...Bb4ch 12.Bd2 Bxd2ch 13.Kxd2! Rf8 14.Kxc2 (or White can try this to reach the ending more quickly: 14.Nxf7 Bxb1 15.Nxd8 Rxf3 16.Rxh8ch Kd7 17.Rxb1 Rxf2ch 18.Ke3 Rf6 19.Nxb7) - White has a winning advantage, but the damage has been somewhat contained. Now Black loses a whole rook.