sorry, but what exactly are runes and how do I get one? (in the guide for the soulstealer (which I play most time) is mentioned that you've to get runes and because of that you should go on mid lane...but what are runes? Haven't seen weired things that spawn on the map as of yet.
1 Rune (A power-up, like double damage, haste, regeneration) spawns randomly every 2 minutes of the in-game time on one of these 2 spots:
The top river spot
The down river spot
Both spots are near the exists/ramps of the Secret Shops to the river, the river being the line of water in that goes diagonal from top left to down right.
It is possible that a rune spawns on a spot where there already was a rune, then the old rune will still be there and no new rune will spawn. Thus it is possible that only 1 rune will spawn in an entire game if no-one grabs the rune. It is also possible to capture the rune with an empty bottle to fill your bottle with the rune and replenish the 3 charges it has.
I would say start a solo normal game and check the river every 2 minutes, you will get the hang of it in no time.