@Madeleine234 I don't know anything about requiremments in Costa Rica, but you should research any permits required, how it works for you to be earning money from a property there as a non-resident, your tax obligations etc.
You will absolutely need someone to co-host for you, and the best way to find someone reliable would be to see if there is a local online hosting group and ask there.
I would not recommend using these big property-management companies- both hosts and guests have experienced serious problems with them, although there may be some small local mgmt co. that is reliable, but make sure you get thumbs up on them from other hosts in the area.
Whatever you do, keep the listing under your profile, not a co-host's or manager's. Listings, bookings, reviews, and ratings, are non-transferable, so you don't want to end up having to start over if your relationship with them goes sour.