you start with 2 rune pages to begin with
there are 4 types of runes: red marks, blue glyphs, yellow seals, and quints.
most people say to only buy tier 3 runes. when I started ranked, I actually bought tier 2 quints for attack speed and AP so that I had SOME stats and that was fine, but if you aren't doing ranked yet just save up for the tier 3. you probably shouldn't do ranked 'till you got some full pages anyway, I was greedy :D
start out with 9 flat AD marks, 3 attack speed quints, 9 armor seals, and 9 magic resist glyphs on your AD page
for your AP page start with those 9 armor seals and 9 magic resist glyphs, and also get 9 magic penetration marks and 3 ability power quints.
I'd recomend just buying the 7 rune page bundle, I did and then slowly filled them all out with more runes.
right now what I use for runes across those pages are, in order of most useful,
marks: flat AD, magic penetration, armor penetration, armor, attack speed, hybrid penetration
seals: Armor, scaling HP, flat HP
glyphs: flat magic resist, ability power, flat cooldown reduction, scaling magic resist, scaling cooldown reduction
quints: attack speed, ability power, movement speed, attack damage, armor. I know some people use gold income on some support champs but I haven't bothered to buy those yet.
so far I haven't had any need for any more runes with all of those, it took me a lot of time to get that many though. at this point I'm just getting champs anymore.
do a google search for rune pages. I have a site I use lots, champion.gg I just search a champ and it tells me the most popular rune page for the champ as well as the highest win percent.