For K2CO3 + 2HCl ---yields ---2KCl + H2O + CO2,
How can I find the # of ml of 6.0MHCl that would be needed?

Your equation appears to be balanced. Therefore, you know that for 1 mole of K2CO3, you need 2 moles of HCl.

If you know the weight, or concentration of K2CO3, you can calculate the number of moles of K2CO3 that are present. Since you know that 1 mole of K2CO3 requires 2 moles of HCl, you can immediately calculate the number of moles of HCl that are required.

You already gave the concentration of HCl (6 moles/liter). Therefore, for every liter you deliver, you deliver 6 moles.

\cancel {moles}\,required\,\div\, \frac {6\,\cancel {moles}}{Liter} = Liters\, required

A mole of a molecule equals the atomic weight of every atom in it added together. That number in grams is 1 mole. Don't forget, if a molecule has 2 of the same atoms in it, that atom's atomic number is counted twice, and 3: thrice, etc...