Shillong is small and the centre is walkable, though steep — Police Bazar to Laitumkhrah is around a kilometre with a couple of stiff climbs. Local taxis are cheap and the standard option after dark. Shared taxis (look for the white Maruti vans) run fixed routes around the city for a few rupees a hop; tell the driver your destination before getting in.
For day trips, hire a Tata Sumo or Innova from one of the operators around Police Bazar — Saiya's, Meghalaya Tourism, or any of the unmarked stands by the State Bank. Rates are negotiable but expect to pay around 3000–5000 rupees a day depending on distance. Allow extra time in the monsoon — landslide closures are routine.