Since you already do 1:12 scale modelling I'll refrain from the obvious steps of cleaning up the mould lines, etc:
a. Select your primer - matte white gives you a brighter overall finish, while matte black gives a darker tone. Black can also be more forgiving if you miss a spot while painting, as the dark colour doesn't attract your eye as well;
b. Select your paint - are you using enamels, acrylics, or oils? I use acrylics because they are water based, clean up easier, and mix fairly well, however they dry quickly, so some layering and wet-mixing techniques are hard to execute. Enamels are harder wearing, but are not as easy to clean up - I generally use an enamel varnish to protect my minis because these are gaming pieces - you may not need as sturdy a finish if they aren't leavign the mantle. Oils are extremely easy to mix and their long drying time allows for quite a bit of wet mixing on the mini, allowing a near infinite range of colour. Oils big drawback is their drying time, which may even take months before it is completely dry.
c. Select your brushes - probably don't need to tell you to get the best ones you can afford - for 28mm scale minis I use 0, 000, and 10x0 brushes for much of my work, and some #1 flat tips for drybrushing;
d. When painting people, I like to start with the skin tones first then work my way out. There are number of good painting guides for minis out there - Painting Citadel Miniatures is put out by Games Workshop and shows a large number of good techniques, So does the painting section of White Dwarf magazine, and also Military Modelling - which also has a larger range of scales and media than either of GW's publications.
The biggest advice I can give is take it slowly and deliberately at first until you get the hang of the scale. Look at your mini from close up when you paint, and also from what will be the normal viewing distance - you don't need to paint what will never be seen.
And most importantly, keep your drink well away from the pot you're using to to keep your brush moist and clean - unless you like the taste of paint in your tea....