Any rune can get you over 12 in an attribute, as long as it's an attribute of your main class. Ie a Ranger/Monk can use a Rune of Beatmastery, Expertise, Marksmanship & Wilderness Survival. But they could not use a Rune of Healing, Protection or Smiting as these are not attributes of their main class.
The max you can get from attribute points is 12, and runes add 1 (minor), 2 (major) & 3 (superior) to an attribute. Thus if you have 12 and a minor rune, you'll get 13 in that attribute.
Note - only 1 rune will add to an attribute. thus if you have say a minor in scythe & a superior, you'll get +3 to Scythe, not +4 - since the higest effect will be counted.
To get runes:
- you can get them from armor drops from monster's. Monster's will sometimes drop armor drops which have runes attached to them (you'll know this as the armor salvage drop will be blue, purple or gold).
- visit a Rune merchant that you can find in most major towns (ascalon city, Lion's Arch, Kamadan, Kaining Center, Shing Jae Monestry, etc). just look for a npc with [Runes] next to their name.. you can buy runes from them (as well as sell runes to them as well).
Also note, if you get the headpiece with +1 Scythe, you can get a max of 16 in scythe (as mentioned).
12 from attribute points, +1 from headpiece, +3 from sup rune = 16.
This does also mean you can get 13 without needing any rune, as 12 +1 = 13
