help me im stupid
im level 10 and i just discovered masteries, and item sets
Item sets frankly don't make a difference. Once you've figured out by a process of observation and experimentation what items are good to build on a champion, you simply won't forget, and if somehow you do, you can just look it up or work it out mentally.
Masteries (as well as runes, but not runes yet) are pretty important. Spend those points.
Generally you're best off if you distribute points in a ratio of 3 to 7 between two of the mastery trees. At your level, this will probably mean that for mages and marksmen you'll want 7/3/0 or maybe I guess 8/2/0. For bruisers you'll want to get 3/7/0 or 2/8/0. For support, it really depends. Get some utility tree stuff and then see if you need durability stats.
At least that's my best guess for what to do at that level.
Don't even bother with runes until you can get tier 3
It's just not even worth it.