Be sure to take bus number 2 and get off at 21st Street.

Literal

Certainly number-2 bus [to-に] riding [and-て] 21st-Street [at-で] get-off [command-なさい].

2番のバスに乗って gives the bus number and action, linked by te-form. 21番街で降りなさい specifies where to exit. Clear travel directions.