Warlock 2024 Phbv. Eldritch invocations — level 1. If you can use a magic item like a spell scroll or a class feature like the archfey warlock’s steps of the fey, you can still cast a spell using a spell slot on that turn.

Your subclass is determined by your warlock’s patron, and while they only start giving you unique features at third level, you’ll want. A warlock could have gained the basic level 1 and 2 warlock abilities from their own research into the occult, coming across an item that instilled eldritch understanding, or worked their way up.