The most general runes you can purchase are armor seals and flat mr glyphs.
They work for everyone and are ideal on virtually every ADC and even a few AP mids.
MPen marks are the standard for all AP mids. As for ADs, it's a toss up between ArPen and Flat AD marks. Heavy harassers, like Caitlyn, might consider the pen, where farmers who go zeal->PD early might want Flat AD instead. Since you're a new player I'd recommend Flat AD, since it will help your last hitting.
For quints Flat AD is the standard for ADCs. Flat AP and MS are probably the most commonly used for AP, but it can vary between different champions and match ups.
When you start to specialize your AP rune pages you'll probably want AP per level glyphs eventually. Seals vary, HP per level, armor, flat mana regen, and AP per level are all fairly common.