when should i put add coolant to my centralA/c unit and is there a way to do it myself?

You should have your equioment checked yearly and as far as doing it yourself you can clean ,you can inspected your unit and duct work and repair damages there but as far as adding freon(Coolant) leave this to a pro the freon is expensive and in my area atleast you must be licensed to even buy it. The tools are also moderatley expensive. If you suspect you are low on freon then have the unit inspected for leaks by a reputable contractor. Normally I completely support diy but in this case I just dont see it being worth it. GOOD LUCK, AC