The rhythm clef is good for Drums, percussions, snare drums, cymbals and any other instrument that can not have a defined pitch.
Allegro means "Happy" in Italian. The tempo is more or less fast, like if you are happy and you move fast, not slow. But remember, all these are just words, in order to understand music you have to feel it. So play a lot of songs that are about 121 to 168, happy songs.
When you play a song, sometimes you play loud or really loud, and sometimes you play soft, or in a low sound, you use the volumen to express different emotions. Piano means "Soft" in Italian, Yes is like the name of the instrument, "Piano" so when you see a "p" you have to play softly, imagine the volumen is from 1 to 10, you will play in about 4 more or less. The dynamic symbols.
Forte means "Loud", in about 1 to 10 it will be about 8. In fact the name of the instrument, piano, it was originally, "pianoforte" because at that time, long time ago, it was the only keyboard instrument that actually could play soft and loud at the same time, it could play "piano forte".
Sometimes you don't play loud or soft, rather you go from soft sound to loud sound little by little, like gradually going louder and louder, this is when you use the term, "Crescendo" or "Cresciendo" in spanish, or "Growing" the sound from soft to loud. From piano to forte.
Try this test to see how much you know so far. You need to know 10 symbols in a row with no mistakes.