what airbrush are you using? what paint? what is your air pressure set at? Are you using reducer in your paint, and if so, what kind? Do you have a filter set up in your air line?
There are just a ton of variables that could be causing the problems, and, since you're new, it could be something simple that you over-looked or simply don't know about, so the more info you can give, the better. It could be using the wrong paint, wrong air pressure, a bent needle, a cracked nozzle, a damaged air cap.... it's a long list.
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If you want to paint t-shirts, the .5 tip is the general "standard". On textiles, it will give a pretty fine line once you get the feel for it. Plus, textile painting usually doesn't require super-fine detail work.
For hard-surface or illustration, I reserve my .5-tipped airbrush for fades, back ground colors, large areas, etc. Generally speaking, the finest line you'll get with a .5 tip freehand would be about the width of a felt-tipped pen.
