Keep trying out new champions. Ask what roles they go in and find out what role you like best. Try buying champions suited for this role. But try to buy one champion per role. Some champions can do multiple roles too so these champions are ideal for starting out a roster. But buy runes for level 20. They cost ALOT. Fill up one page completely then you can take your time a bit on the other page. If you want to go the safe route you can buy the following runes and have them applicable to pretty much any champion you play as:
3x Quints of hybrid Penetration (6150 IP) or 3x Quints of movement speed (6150 IP)
9x Greater glyph of cooldown reduction (7380 IP) or 9x Greater Glyph of Scaling magic resistance (1845 IP)
9x Greater seal of health (7380 IP) or 9x Greater seal of armor (1845 IP)
9x Greater marks of hybrid penetration (7380 IP) or Greater mark of attack speed (3690 IP)
The more expensive ones are safer picks. Something like Greater Glyph of scaling magic resistance is a specialized rune choice. It works better than health against mages but has zero use if you're up against a full team of AD. So going the cheap route means that you will need to buy more runes to cover the weakness of the runes you already own. Which means more pages, which means even more IP spent. So if you don't want to buy lots of runes and are ok having a "one size fits all" rune page then the first runes I listed for each category is the best shot you have at that goal. If you want to go cheap on runes or you would much rather have rune pages that are more specialized and thus reach full potential much sooner then you'll look into the other stuff I listed.