Power supply positioning in PCB design
My general recommendation, based on the very little information you provided, would be to group your power circuitry in one area of the board. This helps in case you need to isolate sensitive circuitry, or separate low-voltage from high-voltage parts of the circuits. Furthermore, some power supplies (especially DC/DC converters) can be very noisy, and you do not want these near high-speed, high-sensitivity signals as it can affect the signal quality. If you look at any professional board, all of the power circuitry is usually kept in one area, and uses an entire layer to distribute the power over the board. It is recommended that you group, say, 3.3V circuitry in one area, 5V circuitry in another, and 12V circuitry in another. This allows you to lay pours connecting each power supply to the circuitry it powers. Try not to route power tracks individually unless you have a reason. 

This is very basic, and by no means should be followed EVERY time -- Sometimes there are reasons why this isn't ideal -- but for a beginner these are general rules you can follow that will help you figure out the proper component placing and routing.