I am now level 6 and adventuring around. I've decided to enter mages guild and doing quest according to this. Also I'd like to join fighters guild to earn some money.I havent started the main quest yet. I found lots of dungeons, forts etc. I entered some but mostly ignore to enter as I dont wanna level up some much for this scaling system.
So, what shall I do? Shall I continue to ths and start mainquest when I lvl up a bit or start it asap??
Starting Main Quest
Up to you entirely 
The beauty of The Elder Scrolls is do what you want when you want(TM). One way or another you are going to encounter the infamous scaling issue, at which point you'll either have to use with a mod or play with the difficulty slider (I'm settling for the latter at the moment).
Basically, if you want to start the Main Quest now - go for it it won't really make a difference, except you might find it too easy too early in the game.
The beauty of The Elder Scrolls is do what you want when you want(TM). One way or another you are going to encounter the infamous scaling issue, at which point you'll either have to use with a mod or play with the difficulty slider (I'm settling for the latter at the moment).Basically, if you want to start the Main Quest now - go for it
it won't really make a difference, except you might find it too easy too early in the game."I fart in your general direction! Your mother was a hamster, and your father smelt of elderberries!"