So you're looking for a trombone? There are certain things to consider before buying. Please note these are my views only and you should always talk to your tutor before buying.
If you're young or particularly small, then you should be looking at a student trombone. I've found the best make for student trombones is yamaha. They produce a crisp clear sound and are much harder wearing than other student models. I've found they are worth paying the little extra for as the slide won't wear out unlike many others. (Mine hasn't and i still use it 8 years after i was bought it).
For the more advanced player, you then have to decide what sort of trombone you want to play: Bass, Tenor or Bb/F Tenor. I play a Bb/F tenor trombone so these are what i'll be advising on. It would be rather inapropriate of me to advise on Bass and ordinary tenor trombones knowing very little about the best models!
A Bb/F trombone is a highly adaptable instrument. You get the full range of a tenor trombone plus a certain amount of the Bass trombones range. The Plug (trigger) also provides alternatives to standard slide positions which can sometimes make playing easier, or possible! For a Bb/F tenor trombone there are many options. Reputable, quality instruments include Besson, King, Yamaha and Conn. My personal favourite are Conn. The 88H is widely regarded among pro's to produce the best sound of any trombone.
The most popular instruments out of these manufacturers are the Conn 88H and the Yamaha YSL 682. Either are superb choices and both will serve you well. I have the Yamaha YSL 682 and the Conn 88H and my best friend has the Conn 88H. Both instuments have their own little quirks. The yamaha is usually better finished than the conn, but the conn plug (or trigger) has a shorter travel distance and a far better tone.
No matter which trombone you choose be it Tenor, Bb/F or Bass, they all need TLC. With this, they should last you many many years. The other most overlooked point for trombones is the choice of mouthpiece. Make sure that you choose the correct bore (size) for the trombone you own. The bore is usually small/narrow (tenor) or large (Bb/F and Bass).
I hope this has been of use to you and if you require any more information or reccomendations for other points of purchase then please don't hesitate to contact me.