Well I have downloaded this game around Friday when I was bored out of my mind. Im new to this whole PC gaming thing and need a little help getting started with the game. BASICALLY i know nothing at all about builds, masteries?, runes and stuff like that any tips would be helpful. 
First and foremost, don't buy (many?) tier 1 or 2 runes. Waste of money to try and fill your pages with runes that are going to completely suck when you get to tier 3. As for what runes to get, runes come in 4 kinds: Marks, Seals, Gliphs, and Quintessences. you can have 9 of each of the first three and 3 Quints. Marks are "offensive" and are generally better at Physical attacking, with only one option for mages. The best marks to buy are Attack Damage(AD), Attack Speed(AS), Armor Penatration, Magic Pen, and Hybrid Pen. AD is the usual option for Physical champions and Magic Pen is usually the option for mages. However, since you're new to the game, running AD marks on your mage might make last hitting easier for you.
Seals are "defensive." The best seals are Health, Armor, Scaling Health(which gives extra health each time you level up), and Health and Mana regen. Armor is probably the best balanced one to pick up for now. Later on, once you get more familiar with the game, you'll want to have multiple rune pages that you can adjust to fit your opponent, but for now Armor and Health/Scaling Health are pretty safe options.
Gliphs are "magic." They're best options are Scaling Ability Power(AP), Magic Resist(MR), Scaling Magic Resist, and Scaling Cooldown Reduction(CDR). The Magic Resist options are better on AD champions, Ap is better on most Mages, and CDR is really a lot more complex. CDRs power changes on a champ-by-champ basis, I'd kind of recommend a new player to stay away from CDR runes unless you know you'll be playing mainly champions that benefit from it.
Quintessences(Quints) are "super runes." They're good at everything, but you can only have 3. Eh... It's kind of harder to give advice on Quintessences, because they also change on a Champ-by-champ basis. As far as "basics" go, AS Quints are good for Marksman, Flat AP is good for Mages, and Gold-per-5(g/5) is good if you plan to play support. You can get more specific information at some sites I'm going to recommend for items.
For Builds, when you open the shop there will be a page of recommended items which will usually be decent. You can get better information on specific Champions at Websites like Mobafire and Lolking These Sites also have Information on what Masteries and Runes are good fr specific champions.
Good luck, and feel free to PM me here or add me in game if you need more help :3
