There is no such thing as being a rubbish artist! If you have good enough eyes and thoughts, then that is all that really matters. Constructing a decent visual language is the hard part, but if you are not a working artist, then you shouldn't worry yourself with things like this!

As an art student for a long time, one of the things I find with people who think they are crap at art is that, although there are some people who just can't see their world in a visually subjective way, the reason most people think they are bad is purely because they have a notion in their head of what something should look like before they have even started! The best works I have done have been near automatic (i.e. free from conscious thought) where I haven't cared about what the eventual product will be but have been more engrossed in the actual progress and methods, taking each stage at a time.

Bear this in mind as an example; If you wish to do a painting of a dog, you often have an idea in your head of what a dog looks likes, or are working from life. You get into the frame of mind that if what you create doesn't look like the dog you are seeing or thinking, then you are obviously not very good at drawing and have no hope of being an artist. But the whole point of art is not to create the same dog out of paint (admittedly sometimes it used to be) but rather communicate the notion of "dogness" through your work; the dog's personality, it's movements, the feeling it evokes in you and so on.

My advice is don't finish before you've even started. I fall into this hole alot and believe me it is never ever productive or helpful.

On the materials front, everyone is right in saying that acrylics are the best paints to start out with. They are cleaner, less harmful, and if you want to change something in the painting they dry very quickly and you can paint over parts. They can also be diluted with water to create washes (the best way to start the painting is with diluted washes/layers first and then build up the paint). Try not to leave any white parts because it doesn't help in the long-term and is bad practice!

Acrylics are relatively cheap, and sometimes you don't even need to buy canvases to paint onto, some cardboards or suitable papers from art shops might be cheaper than mounted canvases. That said you can pretty decent standard canvases now at small prices if you know where to look.

If you want any help then I would be happy to give you some advice!

Good luck!