Rhythms for newcomers
I am assuming that, if you've got this far, you can already read music. If you're new to early music, hear are some of the things you may notice:
Bar lines: much music was written before bar lines came in to use - the bar lines have been added later. So do not be surprised if you come across the occasional bar of the wrong length, for example a 3 beat bar in a piece of music with two beats to the bar. You will also find rhythms which make more sense if you forget the bar line is there, or groups of quavers bracketed together in a way that ignores the beat.
Note lengths: we're used to the fundamental note length being the crotchet or dotted crotchet, with time signatures as 2/4, 6/8 and so on. In early music, the fundamental note is very often the minim or dotted minim, with time signatures such as 2/2, 6/4. (The semibreve and breve are derived from the word for short - a breve is half a "longa"). In the short term you can get by with counting 2/2 4 crotchets, but in the longer term the music flows better and is easier to play if you count in minims, so keep practising until you feel comfortable with this. It follows from the longer note lengths that you will be seeing a lot of minim and semibreve rests. Remember: "Minims mooch on the middle, semibreves suspend from the 4th".
Contrasting rhythms: it's very common for music to switch between 3/2 and 6/4 - without changing the time signature - it's the difference between counting "Why don't we go shopping" and "Not in a million years". It's known as a hemiola.
The hemiola is often only in one or two parts - you find yourself playing 3/2 while the person next to you is playing 6/4, or vice versa. It's tempting to try counting in 6 (1 2 3 4 5 6 vs 1 2 3 4 5 6) but it becomes easier, either by keeping a firm beat going in your head, no matter how much at variance the rhythm seems; or by switching between counting 3 in a bar or a slightly slower two in a bar.
For more information on rhythm, see 'rhythms in early music'.
