Runes are useful because they give your champion "free" stats with the IP you earn. RIOT is also going to put out a sale of generic runes, so if you don't own any of the ones in their post, I say hold of on purchasing those until it goes on sale.
Anyways, if you don't own ANY runes, your best bet is to go for generic runes that will benefit all champions. Flat AD marks, flat armor seals, flat MR or MR/lvl glyphs, and flat HP quints. You can use these on all of your champions, even mages, and it will make your champion strong and buff their early game. The flat AD marks won't help your spells unless they have an AD ratio for some reason, but it will let you last hit with your AA easier. After you have this, I say you should get runes that benefit your prefered playstyle. If you like mages, get magic pen marks, flat AP quints, and consider AP/lvl glyphs/seals. If you like ADC get some flat AD quints. An IMPORTANT thing to remember, though, is that you will need multiple rune pages if you want to customize more. Having all of those runes won't help you if you have to play mage and didn't equip your mage runes into a page.