The most basic systems are a charge and a resource-pool system.

A charge system would be that you generate a charge or "prep" in a single post. Any given ability costs X or Y preps/charges. Expend charges, use abilities. Depending on the ruleset for the community, you may or may not be able to fight while generating a "charge", but in my experience, it's considered acceptable to generate a "charge" as long as you aren't using an ability that spends charges or taking damage, and you're actively acknowledging in your post that your character is doing something to generate said charge (via ki, magic, technology or what have you).

A resource-pool system would be something like a "mana pool" for a spellcaster. I'll use that example, just to stay consistent. They start the match with 20/20 mana points. Each of their spells costs X or Y mana points, which depletes their pool until they reach 0 and cannot use any more abilities. This system also tends to have a use-per-day or a cooldown system attached so that someone can't just spam the same ability every post. They may or may not regenerate a small number of points per "passive" post when they aren't actively using abilities.

That being said, your character comes first. Develop the system to fit the character, not the other way around.