Thunderbolt recently discontinued service between San Pedro and Corozal Town (with a stop on demand in Sarteneja) due to lack of business, but reportedly will resume at least some service tomorrow (Friday).
Tropic's flights from San Pedro to Corozal will stop at the Sarteneja airstrip on demand.
If you're on the mainland you can drive from Orange Walk Town or from Corozal Town. From Corozal you have to cross the New River and Laguna Seca on hand-pulled ferries. Roads are okay except after heavy rains.
There also are Sarteneja buses from Belize City and Orange Walk.
Sartenja is amazing! Candelie's is a nice, inexpensive place to stay. Richie's and Liz Fast Food are good spots for cheap local eats.