Practice on a piece of junk.
Go and buy a $2.50 bag of of those shitty little plastic soldiers and practice your skills on those. No one cares if you mess them up.
Here, these little arseholes
will make perfect guinea pigs for your budding painting talents and torture.
Looks like there's green and silver available. Always go for the lighter colour - silver in this case. It will more easily match the white figures you already have.
Normally, it's advisable to wash the figures first with some soapy water to get rid any grease and detritus that may be adhering to them. Them rinse 'em again. Then you can start with a light base colour and add darker details on top. Using shitty little test subjects like these means you can experiment on them as much as you like - and f**king them over is half the fun! Draw dicks and swastikas on them, too, if you like. No one will care how you treat 'em.
The last thing you want to do is start working on your good figures and mess them up if you are not ready to do so regardless of any techniques people give you. Like a lot of things in life, you will want to practice and learn. Shitty toys can be fun to paint, too! Mess around with colour schemes you would normally be averse to using and practice, practice, practice until you know the result you get is the desirable one.
Then - and only then - will you be ready to seriously apply coatings to those Aliens and stuff you got there.