If you're at Disney Springs and want to get to the parks, you'll need to go to a resort - at least 3 of the theme parks can be easily accessed from certain resorts - only AK will require transferring to a second bus. To get to MK, take the Contemporary bus if you just want to walk over to MK...or the Poly, Grand Floridian, or WIlderness Lodge if you want to take a boat or monorail there. To get to Epcot, take the Beach/Yacht or Boardwalk bus, and just walk to Epcot from there (or take the boat if you don't want to walk). You can also get there by bussing to Riviera and taking the Skyliner. For DHS, the Boardwalk bus is easiest to just walk to DHS - or Beach/Yacht/Boardwalk to take the boat.
Same thing getting TO Disney Springs from a theme park - the easiest would be to bus to Saratoga Springs, get off at the Congress Park stop (#4), and walk over to DS. Or if in MK, you would walk over to Contemporary and take their DS bus, from Epcot you would walk to Beach and take their DS bus, or from DHS, you could walk or boat to Boardwalk and take their DS bus.