Eh... Alright...

What toons to roll?

Basically anything you want. In general you want to avoid specs that are more proc based. Some classes perform better than others (for example hunters are the usual top ranged for us), but I honestly think almost all of them are doable, my personal recommendation is playing specs you enjoy. Roles are up to you.

How to play them?

Depends on the software you choose, the specs you are playing and how much control over each toons individual abilities you want. Look at tutorials for ISBoxer, HKN, or whatever you choose. There's plenty of them around. Similarly Mirai has made a pretty cool tutorial series on multiboxing game play basics, available right here.

How to set them up?

Same as above.

How to level?

Quest and / dungeons. I would say instances are great up to around 100. After that I personally prefer quests, but again, personal preference.