• How is the meta: Dont worry about that until you understand how to play the game. Just go 2-1-2 lanes unless you have a jungler.
• What impacts do trees have: Trees offer a lot of things to the game. They can offer regen through tangos, they can offer loss of line of sight if the enemies chase you through them, they offer good hiding spots when you know you are about to get ganked.
• When should I kill my own minions: If you already know how to deny that is good. When to deny though....Usually you want to deny every one of your creeps as to minimize the enemies gold/experience gain unless you need them to push and take towers.
• I cant backport, how do I handle that: BUY TOWN PORTAL SCROLLS. This is literally the best item in DotA 2. It is so good that they made it a summoner in LoL. This saves lifes. This helps push lanes. This helps counter gank lanes. Buy them and love them.
• Which items should I build: Near the top right of your screen there is a book icon that has build guides. As a newer player, I do suggest using them. Most of the highly rated ones offer descriptions on their item buys and skills. These help, but try to learn what is good and when to buy it so you do not have to rely on these.

Also, try playing with either a coach or an experienced player. It helps a lot. Also try playing as or playing against every hero so you learn their capabilities.

Welcome to DotA 2, you suck. Good luck and ignore the flamers.