There's a lot of specific advice above that isn't bad, and I won't knock it. I'll get a bit more abstract, expanding on a few points.
You'll need to knock out enemy armor. Someone mentioned missile launchers. I preferred lascannon. Whatever. In a 1500 to 2000 point game, my old rule of thumb was seven lascannon. Sometimes I did it with dev squads, sometimes with tanks. I'd scatter around power claws and melta bombs as insurance in case the long range stuff wasn't enough. But you start with just enough anti-armor to get along.... then spend every other point available on anti-infantry.
That's balance. You have the tools to handle what is coming at you. The flip side of this is imbalance. You want to present the enemy with a problem that he hasn't the tools to solve. It doesn't do you a heck of a lot of good to have one fast moving squad, whether it be in a rhino, on bikes, or with jump packs. One fast moving squad will get clobbered before it gets in. If you are going for speed, you need enough speed that the opponent can't kill all of it before it arrives.
Similarly, one tank won't last. The other guy will have enough anti-tank to kill it quick. It is unfortunate, but one approach to the game is to go either all infantry or all mechanized. The opponent's weaponry almost has to be balanced between vehicle killing and infantry killing. If one presents the enemy with either all armor or all infantry, a good sized part of his guns will not have appropriate targets.
You are going with a highly diverse force, a bit of everything. Such a force might be played to create unit by unit mismatches. Whatever the enemy has, you will likely have the right counter. At game time, you will want to look carefully at which of your units tries to match up against which of his.
But some opponents try to win by over specialization, by presenting lots and lots of one sort of thing. They win by army selection rather than maneuver.
One more trade off is quality against quantity. As a marine player, you have already chosen quality. You will put fewer guys on the board than most non-marine armies. (Necron are an exception.) If you play a lot of marine on marine battles, you might learn that it is cool to have higher quality than the other guy. To defeat the other marine's strong armor, it is often cool to go for every armor defeating weapon one can get, to tool up one's leader types as much as possible. In marine on marine action, often the guy with the fewest models has an edge.
Just watch out for the horde army. You might encounter an orc green wave, an infantry heavy guard army, or a swarm of Tyranid. If you go too much for quality, your model count could get way low, and you just won't have enough guns and swords to take out the other guy's superior numbers. Having an ultra mega doomsday gun might let you double the toughness and penetrate the armor of an advancing guardsman, but he is going to have buddies.
Those are some of the trade offs. There is no one solution. From time to time I've encountered a store where people think they have found an optimal solution. Rhino Rush was such a fad years back, armies of nothing but close combat marines in transports. Yet for any approach to playing the game, if enough players start doing the same thing, lots of other players will tune their forces to stop it. I for one always liked to buck the trend, to not follow the popular fad of the day, but to tune my force to give the fad of the day fits.
For now, I would say build up your basic troops. Your numbers are low. I'd also look for one type of model, one aspect of the game, that you are drawn to, that works for you. Emphasize that. If it is something no one else is really into, this might present your opponent with a problem they don't know how to solve. Alternately, it might be because the unit you like is flawed. There might be an easy counter. Asking around before choosing your point of emphasis might be prudent.
Hope this helps!
Bob
