TTC for 4 months. HELP!

I really need some help. My boyfriend and I have been trying for 4 months now. I'm 23 so I figured I would have gotten pregnant pretty easily. After 2 months of trying, I got a box of ovulation tests. I took those for 21 days and I couldn't tell when I was ovulating. So third month of failed pregnancy tests, I start taking prenatal vitamins. I'll be starting month 4 officially in 12 days. If nothing after month 4, we're going to get at home fertility tests. Then after that a doctors appointment to see if anything is wrong. But my question is if you have had this problem before, how long did it take you to get pregnant? What did you do that helped you conceive?