You don't post enough data to derive Kco2.
So, I assume the constant Kco2 as well as Ka (for H2CO3) are to be gotten from a table.
Basically, CO2 dissolves in water (to form H2CO3) by henry's law: [H2CO3] = Kco2 x Pco2
[H2CO3] can be calculated and PH can be determined according to:
H2CO3 <-> H+ + HCO3- Ka