You should know hte formula c=n/v (c is the molarity, n is moles and v is volume)
To change from mass to moles we use n=m/M (n is moles, m is mass and M is molar mass)
We can find the M of KOH from the periodic table (K=39g/mol, O=16g/mol, H=1g/mol)
Therefore, M of KOH is 39+16+1=56g/mol
Now lets find n: n=28/56=0.5mol
Finally, the molarity is c=0.5/2=0.25M
