For the following:

K2C03 + 2HCl -----yields 2KCl + H2O + CO2.

How could I calculate how many milliliters of 6.0M HCl would be needed and how many grams of KCl would be formed in the reaction?

For every mole of K2CO3 you will need 2 moles of HCl. One liter of 6M HCl has 6 moles of HCl in it. Once you work a balance out on the left you can apply it to the right. If your solution requires you to use 1mole K2CO3 and 2 moles of HCl then it will result in 2 moles KCl and one Mole of everything else. The weight of a 1 mole of a molecule can be calculated by taking all of the atoms atomic numbers within it and adding them up.

Example:
1 mole of H2O =
1 mole of Oxygen and 2 Moles of Hydrogen.
Hydrogen = 1
Oxygen = 16
1 * (2) + 16 * (1) = 18g