Final Fantasy X, Sphere Grids?

Alright, I really don't understand much about the sphere grids and stuffs. I played till the end sometime ago, but all my characters seems to be damn weak compared to others. I finished the upgrading of the sphere grid till the end for each character, but I didn't unlock those Level. 1/2/... Spheres. I didn't unlock them because I don't have all those Level. 's Spheres, is it hard to obtain it? Please do help out here, it would be great if you tell me where to get all those Level. 's Spheres easily and those stats like, defense, strength, magic, etc. that best matches for each of the character. It would also be great if you tell me all that I need to know about sphere grids and all those spheres, ... . (;