First time taking birth control :/

Hi everyone. I’m pretty sure the title sums this up but I’ll give a explanation to thorough it out.

I’m in my first real relationship and I’m ready to start birth control. I’ve never taken the pill and I’m actually very nervous. For one, I take lamitical for my mood disorder and my doctor’s office called me saying that my lamitical could throw off my birth control, or my birth control can throw off the lamitical therefor one would out weigh the other. But I was prescribed a certain Mg of Birth Control for that purpose. ( Apri : 0.15 mg/ 0.03mg) I should trust the doctor right ? Ugh I’m trying to get my mental health under control but im afraid to risk it with bc, but I rather not risk getting pregnant . my boyfriend and do have sex a lot and condoms are just too expensive so we agreed this could be a better option. I’m nervous also for the possible weight gain and worsen moods. I already am emotional so I feel like that’ll be tough. Or maybe not ? I don’t know !

But if I do start it, is it possible I take it the first day my period starts ? I find the whole “when to start bc “ process to be very confusing. And if I do take it the first day of my period, am I safe to have unprotected sex as soon as my period is over? We take lots of precautions so regardless I know we’ll be safe. Just kinda looking for some advice/second opinions here. Anything helps! Thank you!