Fuel depends where you live. In UK AVGAS is £1.10+VAT at most airports.

The red lever controls Mixture; the amount of fuel entering the Carburettor jet, and hence the engine for combustion. When you climb high, Above 6000ft, the air thins dramaticly, due to decreased pressure. This reduces the air density. The engine fuel is still being pumped in at the same rate (negligible change in density, it is a liquid). Now then engine functions at a optimum fuel/air ratio, and if you put too much fuel in, the engine runs rough. That is because not all the fuel burns, and the unburnt fuel cools the cylinders further. Therefore by reducing the fuel going in, you can return the optimum fuel/air ratio to give you back a little power. It may only give you 100RPM difference, but at full power that equals approximately a 20% power gain!

Finally, hydraulic brakes are on the tops of the rudder pedals. Push the top of the appropriate pedal to operate the appropriate brake!
References :
I'm an Airline Transport Pilot, and an Instructor

Also, are you renting? Becuase many airports that rent aircraft rent at a wet rate, meaning the price of fuel is already included in the rental rate.

The black "pull" on the control is the throttle. The red "pull" is the mixture. (it controls the fuel/air ratio)
References :
C172 Pilot