I think this would be a process of atleast a few months. If you're skinny, you'll need atleast three months to get stable fat, and actually have fattie requirements(like manboobs, hanging fat etc). If you would gain weight and lose it too fast, you'd have to deal with excessive skin, making you or your mother have to pay a few thousand dollars, not a few hundred as for a new pc.
Get fat? well, eat alot, eat frequent, and make it a routine. Like when you wake up, eat a huge meal, eat a few snacks like two hours later, eat an other huge meal during lunch, eat an other few snacks etc. Thats the only way to get fat on a stable note.
But as I said earlier, you'll regret it, plus, it would take you atleast four months to get minimal results which wouldn't be that impressive either.
ask your mom if she is prepared to pay a few thousand dollars to get skin off your belly.