Things to consider
1 Cut off flash and trim excess plastic/metal where appropriate
2 Some people mount their figs before painting. You can also assemble them but with the plastics you may want to consider painting them on the sprue then assembling them.
3 Prime in white for primarily bright figs, and black for primarily dark figs. IMHO grey is worthless for either.
4 have a plan on what you want to paint a particular figure. Feel free to deviate as you see the effect of painting it, but at least you had a starting point.
5 Take your time and relax. For small areas, exhale before painting (it steadies your hands), paint, then inhale.
6 Paint black underneath metalics, otherwise the effect is faded.
7 Look up online painting techniques such as drybrushing, highlighting and inks/washes
8 Don't be hard on yourself if you initial work is not the quality you would like.
9 Ensure you have a step dedicated to cleanup of the fig (correcting mistakes and the like)
That is my figure philosophy in a nutshell. My final point is this (and this is strictly about figures that I own-I don't enforce this view on others): If you didn't paint it, it isn't really yours.
You can see some of my work in this board. I would rate myself as an average painter to slight above (and I do mean slightly).
