How many BTU I need to heat my swimming pool of 12'x20'x6' from 65F to 70F..
How many Kwh would it take with my electric heater..

I could do it, but I'm too lazy to do that. Basically, you have to figure out the volume of the pool, then from the density (1 gram/cc for water) you can figure out the mass. Water requires 1 BTU per pound of water per degree F. So multiply to get the number of BTUs.

Here's a BTU to WATTS calculator:

Watts/BTU Calculator

KWH depends on the heater. These calculations assume that the system is adiabatic (no heat losses). Normally, you will lose heat from a pool by evaporation, convection, and conduction into the air and ground. Thus, it will take more power than this. To figure out exactly how much power is lost, you need to solve some differential equations or make some estimates:

http://en.wikipedia.org/wiki/Heat_equation