Thanks Alex. I bought a tripod last week that I've been using a lot even on regular set up shots.
Instead of using a release cable, using a timer would be fine? What is IS? Any idea if the EF 75-300 III USM Lens is a fast lens?
I'm a complete noob, I've only been taking pictures of a couple of weeks and don't know much about lenses, but I can't stop taking pictures, I'm hooked

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Welcome to a new photo-addiction
I am sorry to say, but as a total noob handling anything beyond 300mm is very hard .. even 300 will probably be. At least I myself, and I am into photography for quite a while, still consider 300mm not easy to handle properly.
[SIZE=-1]The EF [/SIZE]75-300 III USM Lens is an f/[SIZE=-1]4-5,6 .. that means on the long end (300mm) the maximum aperture is f/5.6 which is not fast at all. If you use a teleconverter this number becomes worse, resulting in a setup which needs long exposure times ("slow shutter speeds") or very bright scenes[/SIZE]. You would be very limited then.
From my experience using a 1.4xTC is fine with an f/4 lens, but for a 2xTC I would strongly recommend a 300mm f/2.8 lens.
The 75-300mm might be an OK-ish lens, but combined with a TC I doubt it will live up to your hopes regarding image quality (
I never used this lens myself though). Image quality always goes a bit downhill with a TC, so if you do not start with a very excellent lens from the start, results might be rather disappointing when a TC is used.
With IS I meant image stabilisation, which comes with some Canon lenses and is nice to reduce the effects of camera shake for long focal lengths.
A timer will do the job in principle, just it is a bit more complicated to get good shots of moving objects. a timer would be of not much use in sports photography, wildlife will be complicated too.
For anything beyond 300mm you need a
sturdy tripod, which adsorbs vibrations well, and you need a good and sturdy tripod head.