Didn't find you in Color Theory/Mixing...
There are a few different things to think about to mix colors, really. I'll mention a few, but don't be too concerned about them now, when you're just starting. Playing with (exploring) the paints, brushes, and paper is best, when you're a beginner.
Eventually you will want to become quite well acquainted with your specific pigments (or 'colors,' to be non-technical and not quite
). Some pigments are staining, for example. This means it is hard to lift them off the paper (to make corrections, for example). This also means if you want to mix your color by painting with one, then 'glazing' with a second, when one of your pigments is staining you may want to use that one first. It's less likely to move as you paint the second on top of it.
Another quality of pigments (paints) is that some have much higher tinting strength than others. So you want to add just a tiny amount of those, as you're mixing. If your pigments include Winsor violet (dioxazine), Winsor blue (also called phthalo or thalo blue), or Winsor green (also called ph/thalo green), these all have extremely
high tinting strength. All three also happen to be very strongly staining, btw.
Another aspect of pigments is that some will just let you get much 'darker' (which we call being high in value, or 'long-valued') than others. In part, this is about the color itself (technically the 'hue'): Yellow, for example, cannot get really dark. Some blues and reds can get quite dark. (A combination of some reds and some greens, sometimes with a little yellow added, will give you a black darker than any black right out of the pan or tube!)
Some people say to mix color, start with the pigment you have that looks closest to the one you want, then add another. For me, it works better to start with the lighter, and then add the darker (wastes less paint that way -- if I start with the darker, I might end up mixing waaay too much paint!).
As to how to get your target color, once you've chosen what pigment looks 'closest' to it: In some cases this is relatively easy. Say you have green, and you want yellow green. So start with some yellow (lightest), then add a touch (tiny, at first) of green.
There are some complications, however. The complications happen because all the pigments have their own peculiar characters. So which green or which yellow you use can really change the result, even when the greens or the yellows look the same! Other qualities of pigments (aside from value, staining, and tinting strength) are opacity (is it transparent, semi-transparent, or opaque?) and granulation (or flocculation or sedimentation). These really affect how a particular mixture looks.
To get browns (which are warm 'neutral' colors, like very
dull reds), or grays (cool 'neutral' colors, like very dull blues), mix 'complementary' colors. More on that when you're ready.
It's good to make color charts, to help you get a good sense of your individual pigments. For now, if Judy's note about Rod's lessons doesn't help, or you want help mixing some particular color, just come on back. Or, start a thread in Color Theory/Mixing, where talking about colors is (almost!) all we do.