Know your size before you make any purchases. Every jewelry store has a set of measuring rings you can try, even if you don't buy anything. You can also use this ring sizing guide pictued below.

In order to print the ring sizer, be sure that your printer is set to 100-percent--do not reduce the print size in your printer 'page setup' area. You can verify that the sizer printed correctly by measuring--from A to B should be exactly 3.5 inches. Once you've printed the sizer, cut it out and cut a slit for slot "A".
With the numbers facing out, wrap the sizer around the finger where the ring will be worn.
Pull tab B through slot A until the sizer is snug, making sure to allow for the knuckle.
The number aligned with slot A is your ring size.
If the measure lands between two numbers, your size is a half size--5.5, 6.5, etc. If half sizes are not available, order the next larger size.