well, what is the animal you're trying to get your hands on?
sometimes parents are adamant about not letting you buy a certain animal because they either don't know anything about the animal, or they know what the media has shown them.
ball pythons are easy to ask parents for because they're super docile, they eat once a week, they are easy to maintain, and they don't get very big.
as it was stated, show that you're spending a lot of time trying to learn about the reptile. if you learn something cool about them, show your folks the website and educate them about how great they can be. i convinced my mom to let me get my first BP by showing her videos of cute ball pythons being docile and handled. i also told her about all the cool facts i learned about them and how interesting they would be. i told her about the size compared to other pythons (3 to 5 feet was still a HUGE snake to her ).
basically, back to the first question - what is it you're trying to buy?
well im trying to get a lot of stuff...red taild boa, GTP, more BPs, and monitor lizerds and stuff and i always talk about reps so ya =\ and my step mom sed my bp was HUGE yesterday haha