1) How rare is Bridger Coins (reply as 1 out of ??? or ?%)
2) How do I complete the first quest, it says i need to be at the Gang World but idk where it is
Each block or mob you kill in either of the mines has a random percentage of you obtaining a bridger coin. There is no set percentage/chance on how "rare" it is. It is random, the chances of obtaining a bridger coin on them are the same, but there will be one you come across that will eventually allow you to obtain it by the amount of times you've mined/killed the mobs.
As for the gang world, you must be the Prison rank "[Free]". If you are this rank, simply type /warp free and click on the grass block in the middle of the menu. This will warp you to the free/gang world.