Birth control.. new to this

So I had a baby 2 months ago. I’ve never taken birth control pills before so maybe this is a dumb question. Anyway, I got birth control pills a couple weeks ago. I haven’t started taking them yet. Does it matter when I start taking them? Is it ok that I waited weeks to take them? Like do they expire ? Lol or do I need new pills for this month? If I start on a Monday do I skip the first pill of the pack that says Sunday? Sorry I know maybe dumb question but please explain thank u my OB didn’t explain much !