Pokemon Style game? VX Or Xp

I Would like to learn how to make a Pokemon style game, but i don't have the slightest idea on how i would go around to get the capture and summon on call Type thing. For example, i do want the player to fight but hes going to be significantly less powerfull than the monsters he will summon.
how would i go around making event-based monster summoning. if thats possible.

If not, could anyone direct me to what scripts i would need?
Thanks,
Wolfturn