In addition to the good tips from the other comment, you can do some small tests by mixing a tiny, tiny amount (like a cocktail stick point, dabbed in the end of the tube) - and making up a sort of "mixing guide" of the colours you have.

Some paint colours have a very low opacity or tinting strength, and others are much stronger. For example, to make a pale yellow, you might want equal parts white and yellow, but to make a pale blue, you may only want 1 part blue to 10 parts white. The blue has a far stronger tinting colour when mixing.

Generally speaking, lighter colours tend to have weaker tinting and darker colours tend to have stronger tinting, though there are exceptions. Another example is it takes the tiniest dot of black to darken a colour, but takes quite a lot of white to lighten it. Also worth noting that mixing in white and black will both "desaturate" a colour - moving it more towards grey.

If you're used to working on a computer, remember that light is RGB (additive colour model), where green and red make yellow, whereas paint is RYB (subtraction colour model), where yellow and blue make green. Basically if you've been digital painting or working in computer graphics before, you might find all your colour mixing knowledge seems backwards.

Anyway, like a small science experiment, do a set of mixes on a board and label them - one part A and one part B, then two parts A and one part B etc - then keep it to refer to in future. You might find it useful to include the single colours on there too - they're nearly always a slightly different colour to what the tube shows. That should give you a foundation of what colours you've got easily available to you - it might also illustrate where you've got gaps in your palette, if you're considering buying a few extra individual colours.

Anyway, for various levels of white skin, you're looking at starting with some quantity of white, a little yellow ochre and a bit of red. You might need a tiny dot of dark brown like burnt/raw umber in there too. Darker skin might start with burnt sienna (a reddish brown), a tiny bit of a dark blue, some yellow, then white and yellow ochre to bring it back up again. After those starting points you're just chucking little bits of other colours in until you get it to the tone you're aiming for.

If possible, it's probably worth having a "target tone" in sight from where you're working - like a skin tone on an existing image, object or your own hand, which you can compare to.

In theory, you can get any colour from red, yellow, blue, white, black - but skin tones are notoriously tricky to get right, so do yourself a favour and start with something "brownish".

Marking out a little mixing guide might seem a bit like "doing schoolwork", but if you can put up with doing it, you'll likely find it incredibly useful to refer to in future.