where do I get cables for front,back brakes?
Go to the bike section at wal-mart. $5 gets you 4 cables and yes, they are plenty good.
how do I mix gas/oil?
I would buy a 5 gallon red plastic gas can, and a 4 once bottle of 2 stroke oil. use the little bottle for measuring, and shake around big gas can to mix 1-5 gallons.
How do I fix small (L) black lever cable to make it tight?
when people dump on their peds, the brake levers hit the ground first, and get bent up. If you get some needle nose pliers, you can often force them to be closer to their original shape.