Building the model first or painting first?

Both ways are acceptable... it is a matter of choice. I used to paint everything before building but then spent lots of time scraping the paint off the surface to be glued together. I have also built the whole kit and then painted after, this works well with tanks and armor in general but some masking of areas you may have painted during the process is in order, such as interiors or decaled dash boards, etc... My preference is to build in small assemblys and paint as I go, (get those small detailed pieces done while they are easy to access) you know, build the engine, paint it, build the frame and paint it putting the finished engine in place, etc... Hope that helps some! The ultimate decision is up to you and if you paint by hand or spray. Just have fun building and you will get better with each kit!

Painting parts before assembly presents the major problem of correcting seams. You will have to putty/sand over areas you've already painted and will have to repaint large sections. That said, sub-assemblies can be painted safely, such as tank turrets, and you'd want to to get the paint under overhanging parts. Tank camo pattersn can be painted after final assembly since these were done after the turret and hull were mated.
Figures may need some pre-assembly painting, depending on the pose. The best figures, such as Pegaso, have such good engineering that they can be painted before assembly with no seams to fill.