My depo shot is due in april won't be getting shot dr. orders don't know the date just mth . when do i need to start be careful to not get pregnant?

Question unclear but: One can get her Depo-Provera after 11-12 weeks, and not have to wait for the 13th week. Depo-Provera is usually scheduled for every 3 months (that's 13 weeks, but getting the next shot at 11 or 12 weeks should be ok too). Life is never exact, so the level of protection gradually drops (not predictable) after 13th, 14th, or 15th week. Just schedule each next shot at 12-week intervals, if worried.