Welcome to VEX!
There’s a lot of fine print with drivetrains, but I’ll simplify it to get you started quicker.
The tank drive is two parallel drive pods connected with a brace. It excels in pushing power, which is great for midfield this year. The X-drive is four wheel pods angled at 45 degrees each, creating an X pattern. This drive excels in maneuverability, which may help weave through the countless objects in Override, but it forfeits pushing power. The H drive is the same thing as the tank, however it has an extra wheel/motor on the middle brace for strafing. The H-drive is objectively terrible though, so don’t do it. Mecanum is also a variation of drive you can use, but it would take too long to explain and I don’t wanna waste your time with my sub-par explanations so refer here for more! If you go tank drive, consider a six wheel variation. Gives you a bit more push. As for wheel size, I’ve never been picky about it, so hover around 2.75” or a bit bigger and that’ll probably work. As for gearing, I’m gonna refer you over here because unfortunately I’m not qualified enough to discuss that (I haven’t tinkered with ratios yet). CAD is one place I can speak for though. If you don’t know, if you’re in a high school, you can ask them to apply you for a Fusion360 Student License, which grants you a free copy for a year. VEX is a learning experience after all! OnShape is good too, but I will always always recommend Autodesk Inventor or Fusion360. Inventor is a bit more detailed than 360 and that’s what my team uses.
Good luck in Override!