I'm going to play a Warlock character starting at 3rd level. I want to know if he can become a lich. If this is possible, is there something that I must do before he can become a lich, such as take some spells or skills? I want to change characters after he becomes a lich, so I only need to worry about the the pre-conditions. The DM gave no limits.
According to one of the items in 5e's DMG called the Book of Vile Darkness, there is a spell that one can learn in there to become a lich or a death knight, but only wizards can learn spells so your DM may (or may not) homebrew a system for Warlocks to become liches.
In past editions, clerics could also become liches, and I see no reason why a DM couldn't houserule a ritual that a Warlock could learn to become one as well. Hell, if it was a ritual any spellcaster could potentially do it, which opens up a lot of conceptual space for a Lich.
Warlocks have a pact feature called Pact of the Book, which allows you to take an invocation called Book of Ancient Secrets that allows to ritual cast any type of ritual spell inside of the book, and allows you to add any type of ritual spell into the book. So yes you can, you just have to add said spell into your book and you are good to go.