What Rick says is absolutely correct. Given that you don't have any experience with maintenance and repair at this point, taking it to someone to check it out is a good idea. However, if your teacher is a euphonium player, then he/she should be able to tell pretty quickly if it's leaking or the valves seem to be misaligned. Here are a couple of other thoughts ...
Very often, the source of a leak on a brass instrument is in the water valve (or in one or more of them if there is more than one). Try just applying some pressure to your water valve by squeezing it closed as you blow problematic notes and see if that affects the problem. Try this on each water valve. The little corks on these need to be replaced periodically since they wear and start to leak air. When that happens, your tone will get "breathy" or "buzzy".
You don't say what the brand of the instrument is. If it's one of the Chinese clones (such as the Mack Brass euph or Wessex tuba that I have), I have to say that I haven't encountered one of those yet where the valve corks/felts weren't absolutely crappy and needed virtually immediate replacement (along with ensuring that the valves were correctly aligned as part of the replacement).
There are some other possibilities, but you should just get someone experienced to look at it since you don't (yet) know what to look for.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)