Plain tickets and dad help please!!!!?

im going on vacation to el salvador in june and coming back july. the only problem is that ma dad doesn't know yet and my mom does. my mom SAS that if i get all the money i need and pay for the ticket i can go and that my grades have to be up all year to go. the only problem is convincing my dad to let me go. (hes very series and get mad easily and hes also suborn) i want to no how much are the tickets and were to get them and if i can get them online.. i also want to convince my dad to let me go but i don't no how to do that i need help anyone please!!!!