General DiscussionPracticing to start ranked

Hi, everyone, I was thinking about getting into the (somewhat) competitive DotA, because I recurrently find people with 3.4-3.7k MMR, and I know I can get better (better farming, ganking, positioning, warding, map awareness, etc).
I have played this game for about 4 years (but, actually, only have 796 games, with 965 hours), because I played other MOBAs (LoL, SMITE, HotS) equally, but never played rankeds in any of those games.
I came to ask some tips about rankeds, or who should I watch for improving my perfomance (I think what I play a lot gankers, roammers, mids and supports). I'm trying to play as much differents picks I can. I think what I should practice more Meepo, because, seeing the forums here, he's a strong pick right now (and I played with him a lot before, so I already know the basics of the pick), but I have problems with ending the game fast enough, so almost always I get to the late game, where most people can lock down and burst down one of the meepoes. Should I search for pickoffs or should I prioritize dealing tower damage when I see some sort of fight in another lane?
I have problems too in breaking high ground, usually ending in my entire team wiped or just never breaking it, so the game get longer.

My common picks for the roles in this game are:

Mid: Zeus, Meepo (only 4 matches actually), TA, OD and Invoker
Support: Skywrath, Crystal Maiden, Lina and Dazzle
Roammers: Riki and Pudge.
Offlane: Centaur Warruner, Timbersaw and Enchantress
Carry: AM, Spectre, Luna and Medusa
Junglers: Enchantress, Axe and Legion Commander

I also played with almost any hero in this game (if you think what I should play another hero instead of one of the written above).

Thanks for reading and sorry for my somewhat bad english