I'm guessing that you've made the cabinet out of pre-veneered board - you do say it's your first cupboard, so I'm supposing you've not matched and laid sheets of veneer from scratch.
Assuming I'm right and the material is man-made board pre-veneered in oak it will have been sanded at the time of manufacture. Still, a little extra prepping is essential, so you should sand again from 120 grit, through 150 grit (if you have it available) and finish at 180 grit. There won't be much veneer thickness to play around with, so your sanding with 120 grit should be no more than is needed to remove any small flaws, such as minor scratches, very light bruising, and so on. After that the finer grits simply refine the surface and take out striations from the coarser grit (120). There's no point going below 180 grit with coarse wood species' such as oak, ash, chestnut, etc because the open texture (voids between grain ridges) of the wood are about the same size as 180 grit.
You can sand all by hand with the grain, or sand using power, such as a random orbital sander (ROS), but the latter will remove wood quicker. Each method has its pluses and minuses, but they both work. In general, if I've worked through the grits to 180 grit with an ROS, I like to revert to a bit of final hand sanding working along the grain using a sanding block wrapped with 180 grit just to remove the swirl marks left by the ROS which can sometimes show up under a clear finish, such as varnish.
Of course, if there are any large divots in the surface, big scratches in the veneer, etc, you'll need to undertake some repairs at some point, e.g., filler or a hot iron and damp cloth to raise dents. It might also be a good idea, assuming this is all a bit new to you, to get an offcut of your veneered board and sand it pretty hard to see how quickly damage might be caused if you're not careful, i.e., sanding through the veneer to the ground. Slainte.