As a band director I suggest to never get a First Act or horn from Costco, Walmart, or a pawn shop (they break). some good brands are Bundy, Conn, Selmer, and Bach. Go with a used horn from an individual who is selling one and ask him/her to play it if they can for you. Plan on about $75-150.
Wisemann and Yamaha both make good beginner trumpets that are not all that expensive. Bach and Getzen also make beginner trumpets in the $100 to $200 range.
Get a Yamaha. they are the best most trumpets are hard to play but the yamahas work great. They also sound the best. they sound like gold. The trumpet is a great instrument to play and alot of fun. Its a very easy intrument to play since there is only three notes.
If you don't know one from another, the best thing is to rent one until he is good enough to choose for himself. Many teachers will rent out an instrument.