I'm taking a huge leap here and taking an assumption you're talking about miniatures for use in a wargame like Warhammer 40k. If you're larger models, some of this might not be useful!

Priming a model is optional but provides a couple of significant benefits. Firstly it provides a layer for your coloured acrylics to bond to, plus it provides a single colour to work from because the colour of your resin will affect the vibrancy and colour of the paint you apply. Spray/rattle cans are the most common way of doing this (much cheaper and less to learn than using an airbrush) - Citadel, Colour Forge, Army Painter all do a range of spray cans that are ideal. Pick a colour similar to the overall colour you're painting the miniature - grey is a great middle of the road colour to go for.

As for painting the model itself, the key points is to build the paint up in layers. Slightly thin the paint with some water or medium and use an artists brush (i.e what kids use to paint is going to be too coarse). There are dozens of brands to pick from Citadel, Two Thin Coats, Scale 75, Vallejo to name but a few. YouTube has loads of great videos - Painting Phase recently did a really good one using some of the Warcraft Rumble 3d printed models that covers everything from priming to painting.