Making a brush

Hey, how do I make my own photoshop brush? I took a photo of a mushroom and after editing it, I basically now have just a mushroom in a layer. I just want to know how to define it as a brush. Is there an option to make a layer into a brush pattern? Thanks.

Oh yeah, I'm basically trying to make a wood-patterned border look like it has fungus on it. So hence the mushroom brush.

Edit --> Define Brush

If my memory isn't failing this evening....