I would also welcome advice on this as a lot of the time I am playing people who don't give me chance to get my pawns, Knights and Bishops in place or even to castle, they just come at me with a nuclear assault in about the 5th or 6th move and I end up losing material and spending the rest of the game trying to defend.
How to be a more aggressive player?

I think you should find your own style, according to your preferences, wich not necessarily must be aggressive. But there isn't anything wrong about playing slugfests, especially for beginners. Obviously you should do a lot of tactics, practice calculation, and try to achieve positions with a great activity of pieces. But if you aren't in the mood...

So,
Punching hard + d pawn + 4 times
= 1.d4!!!
Prescient.
d4 is the battle ground square.
Often Black's ...d4 is powerful also.

If someone is storming you early, they must be neglecting their own defenses. Look for counterplay. Also, there is a chess mentor series entitled something like "Punish your opponent when he goes off book". I've reviewed it three times now and learn something every time. It comes in handy, especially at rapid chess.

At your level, your opponents have a very simple plan -- attack and hope that you blunder. The "winning by blunder" technique.
The answer is to become familiar with the openings, and how to refute bad moves. There are many lessons in the basics in chess mentor which are free. Begginers - tactics, etc.
My second suggestion is that you should not play anyone below 1500. These low rated players will give you bad habits, and you will learn very little about "real chess".

Jelly - you have to try and develop with theats + gain of tempo wherever possible and appropriate. For example if you have a choice of developing move it is usually better select one which has some sort of threat to your opponent's position/pieces rather than a quiet developing move which allows your opponent free choice.
Dave - it sounds like your opponents here are attacking prematurely which is almost always doomed to failure. You should be able to parry any such false threats with developing moves and punish them accordingly. Reading good quality annotated games, master vs amateur books or instructional anthologies or Chernev's Logical Chess will show you how to do this.

For chess tournaments and such; can you train for this or anything?
Punch your opponent in the face... it's a very natural agressive act in chess.

I had a very positional style, where I played the caro-kann and I was glad to take a pawn and defend for the rest of the game. Then I wanted to play more aggresivly, so I studied alot of Morra Gambit games (1. e4 c5 2. d4!?), and played some blitz games with that opening. Now I had to be much more concrete, since instead of being up a pawn, I was down a pawn. That made me more aggresive. Now I play a little more solid, but still very concrete. I can play the open sicilian quite well, but I am still happy if I get a position with a small edge.
So I would say: Try to play more aggresively, but play the best moves! I have seen alot of players who plays "aggresive", with the meaning that they push down their g-pawn, and around move 20 they are dead lost. I believe that playing aggresively, but still playing sound moves, are the way to improve.
So I would say: Try to play more aggresively, but play the best moves!
Oh, I didn't know that it's that easy!

Get a few model games featuring common mating attack patterns. For example against a finachetto pushing the rook pawn down the board. For regular castle different sacs on f7 and h7. Solve a lot of mate puzzles involving a castled king. For hard puzzles you don't even have to attempt them, just play over the moves to learn the patterns.
Then (or at the same time) learn an opening like the Barry attack or 150 attack or some gambit lines (King's, Latvian, Danish, Cochrane) and just play them every time lol. Or otherwise opposite side castle every game, point everything at the enemy king, and go for it. This is the process a lot of very aggressive novice types have gone through to get that all or nothing style.

So I would say: Try to play more aggresively, but play the best moves!
Oh, I didn't know that it's that easy!
If you mercilessly attack your opponent's king and scrupulously take care of your own, you should do fine.
Glad I could help.

Punch your opponent in the face if he has one... it's a very natural agressive act in chess.
For chess tournaments and such; can you train for this or anything?