@Lik...what you do is make a new document the size you want the squares of the grid to be. (ex: 30 x 30) And make it transparent.
Then select all and use "stroke" on it with the color you want the grid.
After that, save it as a pattern. (Edit...Define Pattern)
Then make a new document the size you want the whole grid to be. Fill the background with whatever color you want. Then use the bucket fill to fill the grid pattern you made.
FINITO!
If you can't figure it out then hit me up on the PM and I'll walk you through it more detailed.