Get in touch with the RSPCA or the Blue Cross as soon as possible to get some free advice. Cats need jabs to stop them getting horrible diseases, and should be spayed/neutered to avoid repeats of the situation you’ve just found yourself it – those organisations can help cover the cost or put you in touch with other people who will help.
You probably want to get a litter tray and some litter unless you want your garage to stink of cat urine. Balls of paper and bits of string make good free kitten toys. And let the poor thing inside, cats are friendly, sociable animals despite what some people would have you believe, and s/he’s probably feeling a bit lost and lonely…
And yeah, post a picture!