Hi 202. Welcome to the forum. If you have the M42 there are many many mods you can do, that aren't expensive. You have some awsome parents since your only 16 and your first car is a BMW. First, the throttle cable... no touchie. This isn't a carburated (I hope I spelled that write

) engine. If you mess with the cables in a carburated engine (pre 80's I believe in the BMW's) you can mess a little with the fuel/air mixture.
The throttle cable in a fuel injection system can be adjusted if you have play in your gas pedal. Lets say you are sitting in your driveway and you lightly tap on the gas. If the response (known as throttle response) isn't fairly quick, your throttle cable may be loose. If you tighten your throttle cable at tis point you will make the time from when you step on the gas to when the throttle body opens quicker.
Honestly I am going to start out by agreeing with the others so far. Get used to your car. Honestly these vehicles running stock are quite fun. I'm not going to preach any on youth driving, but others will probably agree with me that an experienced driver can have his hands full with mods on these engines, let alone someone who hasn't driven for long.
One of the important things to do before you just start getting things is take stock in what you have. Is it a manual or automatic.
Does anyone know when the 3 series went to OBD2?
One thing to do to find out more information on your car... call the dealership, give them your VIN and they can tell you when size, model and other information about your engine. Everything you would need. If you have an M42, there are plenty of mods you can do.