Go to a fragrance or perfume store to try out the perfumes they have on display. Try all all of the testers. Something to remember is that after even the second or third fragrance, our sensitivity towards the smell of the fragrance decreases. It would be better to leave the store for a while, and come back after your nose has gotten back the sensitivity to detect the scent differences. Most people say that spending too much time in a fragrance store hurts their head so be careful about how much time you spend in the store.
Ask for a sample of the perfume you want to try. Put some on a pulse point (wrist, crook of elbow, etc.) and walk around the store for 20 minutes. Smell it again, and then make your decision.
Think about your personality. If you're outgoing, go for something fruity. If you're more of the shy type, try something cinnamon.
Another option is to go by season. Something watermelon would go good in spring or summer while something cookie-scented could go good in winter.
Go by occasion. Wear something a bit romantic if you are going to a wedding! If you're just going out to the mall, do the above step.
Ask for help if desired. The saleslady is not meant to just stand there. However, don't let her bug you. See Warnings!
Compare prices to get the best deal. The best way to do that is to look at some online store prices like Ebay, Nordstrom, Macy's, and similar places. The prices of perfumes of these big store name brands are much higher than at cheaper discount websites.