How to buy new stuff
There are stores spread all around the world in SL. The easiest way to find stores is to click on the Find button at the bottom of your screen, click the Places tab, and enter a search term in the Find field.
Clothing varies widely in price and quality, so shop around before spending a lot, to get a feel for who sells good quality for fair prices.
To buy an item, you typically just right click on it, and select Buy. But if this is not available, but the Pay option is, then you are probably dealing with a scripted vending machine (see next question).
Many people who sell things in SL are quite helpful if you have any questions about an item for sale, just send an IM to the person who owns it.