How To Heat Up A Pool?

Uncover the tarp and fill it up with clean water, so the sun will warm it up for you.

i hate to tell you this but you cant heat a pool without a heater...running a hose from your hot water heater wont work because the cold water in pool witll cool that water faster than your hot water tank can cook up more..contact some pool cleaner companys and see if you can get a used heater from them for cheap..they are always taking them out ad replacing them with new ones..put a wanted ad on creigslist

You would be better off running a garden hose from you inside faucet and filling it with hot water. Granted even a small pool would easily be an entire water heater full.

heating a pool even a small one is going to cost 

You did not give us enough info like how cold it is going to get wherever this pool is. No money makes things real difficult becouse even solar systems require some money to make them efficient enough. Then, depending on that sort of system and pool size and such, you will still be lucky to bring it up above ambient temp anyway. The most reliable would be a gas/propane, electric pool heater.  Even then you may want to try to enclose the pool too, again more money.

I want a million dollars.

there is no cheap solution.  heating water takes an amount of energy defined by the volume of water to be heated times the temperature rise required times the amount of heat lost ... the the operating energy cost and it can't be avoided.  If you are determined, look for a used solar hot water system but installing it is a chore for even a seasoned DIY guy

You don't say how big the pool is, but they do make solar heat blankets for pools which are nothing more than heavy duty Bubble Wrap in big sheets.  They can do wonders on a sunny but cold day.