GFCI with 12/3 line and 12/2 load
There are basically two options:
New Breaker
This is the simplest option, though probably not the least expensive. And may not even be possible, depending on the breaker panel. Replace the existing 20A double breaker with a GFCI 20A double breaker. If you can do that, then the wiring is easy:

Blacks together
Red to hot on new (ordinary) duplex receptacle
Whites (neutral) together and with a pigtail to neutral on new receptacle

GFCI Receptacle
This gets a little tricky, and also depends on your dishwasher circuit not required to be on a GFCI circuit. According to this page of code changes this is required as of the 2014 NEC. Which according to this page of NEC adoption by state means pretty much everywhere except Kansas and Indiana. There may be specific exemptions in some areas as well. So if it is allowed:

Blacks together
Red to hot line on new GFCI duplex receptacle
Whites (neutral) together and with a pigtail to neutral line on new receptacle
Load side of new receptacle is not used at all.

The reason you can't use the load side, which would provide GFCI protection for the dishwasher, is that you can't have kitchen countertop receptacles shared with hardwired appliances (with some very limited exceptions).
If you could put in a two-gang box then there would be a compliant with newer code solution:

GFCI receptacle on one hot
GFCI dead front on the other hot, wired to the dishwasher

GFCI devices have to be accessible (so you can reset them), so you can't just bury it behind the dishwasher.