The definitive answer is contained hear Mormon.org/values/missionary-work
but basically here are the basics
all worthy men 18-25 go out for 2 hours and serve away from home they do this full time
young sisters of a similar age do to
when retired a couple may go on a senior mission and serve for any time from 6 months to 2 years and may do so repeadedly young missionaries only serve once but may serve again when retired
there are also members who are called in their local area to serve in their home area
all mission work is self funded