It's so simple I won't grudgingly point you to GameFAQ's. Basically, melee damage (Tidus,Auron,etc) increases directly with STR. STR can be found in Tidus's Sphere Grid, Auron's Sphere Grid, and Wakka's Sphere Grid.
For magic spells the MAG stat increases damage directly. MAG can be found in Yuna and Lulu's sphere grids. Rikku and Kihmari have kind of odd grids, but Rikku can be used as a balanced grid if you feel the need to expand. (I took Yuna through Rikku's before I took her through Lulu's).
Summons derive their stats directly through Yuna's increased stats. For increased melee damage, give Yuna more STR. For increased magic damage (as well as special moves) increase her MAG stat.
The following FAQ will give you in depth (more than you'd ever want to know) information on statistical mechanics:
http://db.gamefaqs.com/console/ps2/f...sy_x_stats.txt