A good introduction to the euphonium would be a three-valve model, like the Jupiter 468 Series 3-Valve Euphonium or the Amati AEP 231E Series 3-Valve Euphonium. These are simplest format of the instrument, allowing for a straightforward experience learning the basics. Then, once you've mastered the three valves, you can add a fourth with a horn such as the Allora AAEU-204 Series 4-Valve Euphonium or the Amati AEP 345 Series 4-Valve Euphonium, to further expand your skills and prepare for advanced and professional-level compensating euphoniums.
Along with the valve count, you can also take your pick of instrument style when choosing from these student euphoniums. For example, the Amati AEP 231EC Series Convertible 3-Valve Euphonium can be converted with a mouthpipe swap into a marching layout that's easier to play on the move. Others, like the Yamaha YEP-211 Series 3-Valve Euphonium, use a frontal position for the bell that allows you to project the sound in a more targeted way than the standard design. Any one of these euphoniums could be the best choice for you, and it all depends on what you're looking for from your instrument - so go ahead and explore the whole section! When you find the right horn, you'll probably know it.