The euphonium is a conical-bore, tenor-voiced brass instrument. It derives its name from the Greek word euphonion, meaning "beautiful-sounding" or "sweet-voiced" ("eu" means "well" (understood as "good" or "pleasant") and "phonium" means "voice"). The euphonium is a valved instrument, and nearly all models are piston valved, though German rotary valved models do exist (e.g. Miraphone).
The euphonium has a fundamental pitch (all valves open) of Bb, but sounds in the key of C (relative to the piano). The technique involved in playing the euphonium is quite similar to that of the trumpet, in that they fingerings and partials are more or less the same—disregarding register and transposition. However, the embouchure is much more comparable to a trombone embouchure and oftentimes the mouthpieces are used interchangeably. In the United States, the Bass Clef is generally accepted as the norm, while in Europe it is much more common to read music in Bb Treble Clef (as is written for the trumpet).
Instructor: Jonathan Chasteen Edit
If you are not already acquainted with elementary musical notation, please review the Wikipedia page on Music Notation. I will not be going over certain things, e.g. the basics of musical notation, so it will behoove you to know at least the elements of the musical staff.
Lesson 1 - Embouchure and breath
Lesson 2 - First notes