Why do my muscles tire out really quickly (calves, forearms too) when working out?
Are you referring to the burn? If your muscles tire quickly, that is a good sign that you have more improving to do. Push through the burn and continue to push yourself. Your tolerance for pain will increase as will your strength and endurance.
Make sure your diet is giving you enough nutrients to supply the energy you need to exercise. Make sure you're drinking enough water!
Happy to help with more specifics!