I just hit level 20 and I want to start getting runes. I know is hard to choose one set of runes that will fit most of the top champions, but I want to use one that I can use for my main tops (Nasus and Jax, but planning on trying more).
So I want to choose one set of runes good enough for an average top laner, before choosing a champion.
Any ideas?
This will be tough to answer as all my top champs have 2 different rune/mastery pages.
When it comes to top, you really only have a few things to keep in mind:
With nasus literally all you have to do is stack your Q all game, you should average 100 stacks for every 10 minutes. If you can't manage that in a game you are getting beat. You dont have to fight any1 you dont have to push literally just sit top and get 300-400 stacks then start ganking lanes and 2-3 shot your opponents carries. Nasus can typically take any top laner at or above lvl 6 with 100 stacks, although if they are any good they will just poke/kite you down pretty hard.
With Jax, I don't know his 1v1 mechanics very well as he is a champ I dont top with. But he is strong after level 4, then typically snowballs if he gets any kills.
I would say your best bet for an all encompassing runes/mastery page for top would be:
As for masteries, just go defense and maybe 3 points in utility for the extra Movespeed.
Would be better if you use 2 pages so you can have atleast an AD champ/AP champ rune page.
As for masteries go 9 in damage then defense with the 3 points in utility for MS.
I am a big believer in Movement speed as a top laner, it makes dodging a lot easier. You can outrun ganks and save a summoners and you can either kite easier or make it harder for people to kite you. If I am snowballing top I will always get swiftness or mobility boots so I can chase down carrys and wreck late game. As it will typically get more and more challenging to get to carries as the game progresses.
Also a thing to keep in mind is, don't waste your IP on tier 2 runes. Just get tier 1 till level 30. If you are saving you should have enough to get atleast a couple full pages right away.
I think that for top lane the most important rune is the Movement speed quintessence:
Reason: you have to run a lot because top lane is far from dragon, you'll probably get ganks and since you are alone you have to grab every option you have for survivability. I know that these runes are not cheap but they worth the price.
As for seals you have several options but the most important is the Armor seal: 
Reason: Most top champions are doing physical damage and even the ap champs can poke you using auto attacks (think about Nidalee for example).
In the glyph slot you should put Scaling (or flat) magic resist runes: 
Reason: There is no other all-round option. You can take hp or ap, but with magic resist you can't go wrong.
The mark slot is the most problematic because there are a lot of useful runes for this and depending on your champion you should take the optimal one. My most used runes for top:
To sum it all up:
If you have decided to focus on top here are the must haves:
Check out this detailed list about runes.
You can see that not all runes with the same stat have the same effectiveness. For example the most effective armor runes are the armor seals. In that table the most effective rune is highlighted for each stat.
