also mask anything you dont want to get paint on or that you already have painted another color to keep that color, ESPECUALLY CLEAR PARTS! for this you can use regular masking tape but they have specualty masking tapes for modeling, i recommend the Specialty masking tape beacuse its usually easier to cut and stuff, regular masking tape at times can be thick. i use regular masking tape most of the time beacuse its on hand and easily accessed, but it can be a pain.
also fill in cracks and seems of the model like parts that don't quite fit together like you want them, there are many Methods for fillers, like squadron putty and such, my favorite method is Bondo, i have used it on multiple projects.
as for applying paint, it depends on what type of paint you are using as many have stated, using Acrylics make sure your model is nice and clean and such before application, if you dont clean your model, sometimes there are oils from factory molding and even from your hands that get on the model and the paint probably wont stick quite so well. XD
hope it helps some of the stuff i said some people have already stated probably.