when is it safest to have sex?

my boyfriend and I have recently started talking about having sex because we have only been together for 4 or 5 months and it’s both of our first relationships ever so we’re taking it kind of slow. but idk if you’re supposed to have sex during the fertile window or after it or before. can someone please tell me when it is smartest to have sex if you’re trying to avoid getting pregnant? i know you should definitely use protection so that’s what we plan on doing but also, i’d feel way more confident that i wouldn’t end up pregnant if it was at the right time during my cycle that gives me the lowest chance of getting pregnant😅 i feel soooo dumb asking this but i’m also 18, almost 19 and a virgin who is dating a virgin so i am very new to this idea😂