Cap,
Welcome aboard!
The main thing I'd say is you don't need to buy a whole bunch of paints up front. If you wanted to paint every unit in the game, yes, but start small.
For a beginner painter, I'd recommend making a decision - "Is there one faction I really like, or am I happy to do generic camouflage schemes?"
Let's say you want to do camo. Then I'd recommend starting with medium green, medium brown, black, white, and metallic paints - acrylics, of course - and either some "Army Painter Magic Dip", or - it's cheaper direct equivalent - a small can of teak varnish ;)
Wash the mini first - removes any oils or other things left over from casting - and then paint it all white. This is priming, and helps the colours stick, and come out better. Then base coat in one of green or brown. Add stripes, patches, whatever of the other colour. Do cockpits in black, and any bits you want to emphasise - joints, gunbarrels, heat sinks - in metal. Then brush on a layer of the teak varnish. (Note: acrylics come off your brushes in water, but the varnish needs turps. Varnish also takes much longer to dry.)
That's it for a first up! You'll be suprised how effective this comes out.
Then, pick up a sand or tan paint. Tan and brown makes mountain camo, tan and green for plains. Maybe pick up a faction colour - purple for the glorious Free Worlds League, red for the admirable Draconis Combine, jade for the "they try harder" Capellan Confederation, & so on. Paint some panels, or features, in those colours to show allegiance.
There's some very basic ways, using limited paints, to get some really great results - what we call "table ready".
The reason for having several shades of the same colour comes down to more advanced painting techniques. Bone white is slightly yellower than flat white, for example. Have a look at
www.camospecs.com to see what people who've been in the game for a long time can get up to. But for a beginner, don't stress it. Do what you like, try what you want to do, and - hard-won advice from someone who works at 'table-ready' via the cheap shortcuts - avoid yellow and white until you're ready ;)
Cheers,
W O0