If you want to get into boxing for fitness then look for a local club and they will have trainers how can assist you on a fitness program. Dont believe you will climb into the ring for sparring and do ok as you will get embarrassed badly.

As for your question boxing forms, i presume you mean fighting styles such as Outfighting where you work from range behind your jab or infighting where you look to get close and hit with mutliple punches.

If you mean defensive guards such as Peek a boo or the philly shell then these are things a proper boxing trainer will teach based on what he believes your ability to be.