What subway do you take from the Megabus stop at sixth Ave and 28th St to get the J train to Jamaica Center?

Walk down to 23rd Street and 6th Avenue, and take the Downtown F train (of the B-D-F-M, the orange line) from 23rd Street to Delancey-Essex Street.

Transfer at Delancey-Essex to the Uptown (Brooklyn and Queens-bound) J train to the end of the line, Jamaica Center.