You should probably play as a human until you understand the game a little better.
With the human team, some skills, such as aiming, can be carried over from other FPS's.
Speaking of aiming, it's best that you join a server with a low ping. Basically, it's a measure of how long it takes a server to respond to button presses and such. You are required to lead a target more and more as your ping increases, so try to join a server with a ping of 100 or less, preferably less.
In my experience, armor is usually more important than weapons. With better armor, you might survive long enough to kill an alien. If you skip the armor and get a better weapon, you will find yourself getting killed in 1 or two hits.
As for weapons, it's not always best to buy the most expensive weapon you can afford. The choice of weapon should depend on the circumstances at the time, and then what you can afford.
A good example of this is the mass driver. It's somewhat expensive, but it's utility is rather limited. It's Tremulous's sniper rifle. If you take it into close range combat against a Dretch, you will probably die (unless you have leet skillz). If you are defending a long hallway, you might do a little better.
And then there's the shotgun. It's cheap, and does good damage, but the rate of fire is low, and the range is terrible. It's a terrible choice where distance is involved, and if your aim is poor, the low rate of fire won't always allow a second or third shot against a dretch, although the spread can be nice.
Also, make sure you use your medkit if you get injured badly enough, or if you are running back to base to heal. If you use it, just step onto a medipad to get another one.
If you are injured or low on ammo, a tactical retreat may also be in order. Each time an alien doesn't kill you is one more kill they need to get to the next stage.
It's widely accepted that a mouse and keyboard is faster and more precise than a controller. It may be difficult to adapt, though.
