the guy at the pet store doesn't know what he's talking about. Often, snakes are sexed when they are in their first week. The only time a snake cannot be sexed is when it is still in the egg. Find a locally owned pet store that knows how to sex a snake and ask them to do it. You could also bring the snake to a reptile show and ask anyone who works with large constrictors if they would mind probing your snake to determine gender. Most of the vendors at shows are quite friendly and extremely helpful. If you're only keeping the snake as a pet, the gender won't really matter. If you decide you want to breed it, then you would need to know so you can find it a mate.
Another option would be to take some very clear photos of the tail of the snake, especially underneath, and post them in the ball python forum. I'm not too familiar with them, but many species can be sexed visually by experienced breeders.
As far as feeding, select a food item roughly the diameter of the largest part of the snake's body. I always use something a bit smaller in girth. Get frozen mice or rats from your local pet store and thaw them out with warm water. Once COMPLETELY thawed, feed your snake by wiggling the thawed rodent with a pair of tongs. Let your snake set the feeding schedule. It knows when it's hungry better than we do. Give it a few thawed mice or rats. When it's no longer hungry, it'll lose interest.
If the snake is small, it might eat 2-3 times a week. As it grows larger it's metabolism will slow down. My brother's ball python only eats every couple of months.
Once you figure out what size rodents your snake can eat and how much, I would advise ordering a bunch of frozen mice and rats online. You can get enough to last months or even a year and keep them in your freezer. It's much cheaper and more convenient that way.
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