Wobble in the Desk I am building
How do I rectify this? This is my first project.


That's an easy, inexpensive and useful first project.

Even if the work is square and accurately cut, it's easy to be off by a couple millimeters. And even if you're not, it's easy for the floor to be off a couple millimeters; tile is not perfect.

Your best bet is to do a web search for furniture leveling screws.  You screw them into the feet, and by adjusting the depth of each one individually you can dial it right in to not just remove the wobble, but also get the work surface exactly level in both directions. They'll be fifty cents or a dollar each, or thereabouts.