17 and ttc

Hey I'm 17 and ttc. And I just need some advice and help.

So I was class of 2k17. I've already moved out of my Mama's house and am living with my S/O. We are getting married on exactly my 18th birthday. We decided to ttc because I asked. And he said he always wanted to have a baby with me. So for about 3 months we've been trying. I haven't got a positive. I've been told that I need to lay on my back after sex for 30 minutes. Or with my legs in the air. Any suggestions?