Lara,
Interestingly enough I taught a "Music Fundamentals" course at UMN last year. If you're looking for an online site to "quiz yourself" on intervals and chords, check out this site:
http://www.musictheory.net/
My advice for intervals is to use a "tune" to help you remember them. Associate the beginning of a familiar melody with all the interval types. For example:
Unison: obvious...it's the same note
m2: Jaws
M2: start of scale, or "Happy Birthday"
m3: "Greensleeves"
M3: Sounds like the start of a major triad, no good tune here.
P4: "Here Comes the Bride"
TT: "Maria, from West Side Story"
P5: Star Wars, Thus Spake Zarathustra (or 2001: Space Odyssey)
m6: don't have a good one, but the old Tim Burton Batman Theme sort of works
M6: "NBC Theme, My Body Lies Over the Ocean...
m7: Ford Theme (Have you Driven a Ford Lately?)
M7: sounds like it has to resolve up to the octave
Ultimately, pick tunes that YOU like and can remember. And this would work a lot better if I were actually at a piano and could play the melodies and show you what I mean. But, the bottom lineis, quiz yourself...a lot.