which is the leading hand????

i am just begining to fight (box) and one question has me kind of confused. when you are fighting southpaw which hand is your leading hand? and also what is the difference in a jab and a straight right, i know it sound like i am clueless but those are the only things that i had questions on, besides that i am doing very well. ok thank you for reading and i hope yall can help thanks in advance!