If I have a sprint phone...?

[hi im mya! =]]

And I bought a phone from somebody off craigslist could I cancel the phone I already have and use the one I got off craigslist? Sorry if this is a stupid question to some.. I just have no clue about any kind of this phone service switching crap. lol
Oh and the phone that I bought of craigslist is originally qwest phone & I do not have a SIM card to switch between the two....