What is a good exercize to strenghen my calf mustles?

Calf raises. Put a rucksack with some weight over your shoulders, hold your palms onto a wall or something similar and tip toe. Nice slow movements, will do. ;)

Well I just started running barefoot.

With shoes I was running up to three miles, and since I switched how I was running I only did 3/4ths of a mile.

My calves KILLED the next day. I didn't even feel it that badly while I was running, but the next day it was hard to even walk.

If not then just going up and down on your toes (calf raises) are good, especially if you add weights.

Good luck!

Calf raises but lots of plyometrics are the best.

Walk up stairs using mainly calf muscles.

dont overdo it.

Sit on a chair and lift your heel up. Trying to keep your foot like this, push your leg down. (Static tensing) Keeping this resistance, out your heel down, then bring it back up.