First you open up a new image no bigger than 100 x 100 pixels

flood fill with any color or pattern that you like

play around with the  texture effects such as weave, soft plastic, straw-wall etc..

 

when you are pleased with your tile then lets make it seamless

go to your selection tool draw a rectangle don't get to close to the edges kinda center it

 

this will have the marching ants around it

 

go to selections at the top of your sceen:

scroll down to where it says "Convert to  seamless pattern" click that and you now have a seamless pattern tile