Anywhere between 10 and 30, pick up the secondary class that you're going to need to unlock your job (Lancer in your case for unlocking Monk) and get it to 15. Marauder is the other class you'll be able to pull cross-class skills from as a Monk, so it's also good to pick that one up and at least begin on it. In MRD's case, the majority of its cross-class-able skills can be earned in just the first 8 levels and that much is really quick and easy to get, so you may as well do it early on and have access to those skills while you continue leveling. At some point, you'll want to get Lancer the rest of the way to 34 and Marauder the rest of the way to 26, but that can wait until later if you want. I'd aim for PUG 30 / LNC 15 / MRD 8 for now.

Other classes are optional for if you want to branch out into other areas. It can be done any time you feel like, but I recommend trying out all of them for at least a little while.