There are lots of factors in the situation that could be why you haven't conceived. Were you on birth control before trying to conceive? Tracking your ovulation with opk's (ovulation predictor kits) and bbt(basal body temping.
Ovulation Calendar and Ovulation Chart - FertilityFriend.com) can help you to determine when you are ovulating, to help determine best time for conceiving. Staying in bed after intercourse for 20-30 minutes, with your hips slightly elevated may help. Don't douche as this will wash everything away. From the back, or missionary style are also the best positions for optimal "shooting range" ;0). You are only 20, and have lots of years ahead of you to keep trying, don't doom yourself to infertility yet. It takes most healthy couples up to 12 months to conceive naturally, before it is suggested that they seek medical help. If you factor in past birth control used, it could take longer. (the depo shot for example can take up to 18 months before you become pregnant again.) a lot of it comes down to plain luck and timing. Stressing over becoming pregnant can also cause it to take longer to conceive. Once you have been trying for 12 months, go to see your doctor. There could be medical reasons for not conceiving (on your end, or your partners. Low sperm count, blocked tubes, endometriosis, etc.) you might need some help with clomid, or you might just need more time. If you are not taking prenatals at this time, I suggest you start. At least take folic acid.