What do I do to prepare for painting models

So im a teenager a tad bit irresponsible, but i want warhammer the thing is my dad says i cant paint them since i have little experince painting since i only painted one model spitfire and the spitfire i painted turned out kind of bad, so i was wondering anyone who paints warhammer, what did you guys do as newbies to prepare to paint them because my dad says i have to paint more plane models to get used to it but plane models are expensive and i need all i can get to buy the warhammer models due to their incredible price.

Ps Im making a powerpoint to make them understand, and also i want thousand sons so if you are a thousand sons player please give me advise on how to start.