Before starting remixing/composing in a sequencer you need to find some good audio samples. Well, finding them is not the point; there's alot of sites offering free clips.
The easiest sort of audio may be drumloops, because in most libraries they're categorized in styles and tempo. So it's typing e.g. 'techno 140' into their local search engine, and there you are. But...

...sometimes you'll find a nice loop without knowing about BPM.
Or you find a clip which has nasty starting/ending gaps (doesn't flow seamlessly).
Or you find a whole song and you just want to use its intro part, let's say the second bar, where the drums have fully faded in and play their pattern solo - and you might wish to cut this bar for your own drumloop library.
Ok, assuming a standard kick-snare-kick-snare-drumriff, here's how I do it...

1. Load the stuff into Audacity.
2. Zoom in at the desired area.
3. Select the desired bar(s). Look for peaks, in our case the start/ending is very near before kick peaks appear. Kick peaks are normally lower (and due to their deeper frequency have wider waves) than snare peaks.
4. Again zoom in - at the starting position, until you see the discrete sample steps. Correct the marker position: a good place would be a zero crossing, some milliseconds before the peak attack starts rising.
Then zoom out until the end of your selection comes into sight.
5. Zoom in - at the ending position and correct the marker position as described above.
6. Playback your selection in Loop Mode (Shift+Play). Listen to it - isn't it sounding right? Isn't it seamless? Is a Hihat beat missing? Maybe you have to drag the starting/ending of your selection to the neighbour peak. Sometimes it isn't that easy to distinguish kicks, snares and hihats from each other, only by seeing the waveform. Don't mind, that's normal, just drag your selection's borders until you captured the desired portion.
7. Ok, now the loop sounds right. It flows without (hearable) gaps or jumps.
Take a look at your selection's length (at the bottom bar). You may read: 1.936 seconds for example. So what's the BPM?
8. Open a BPM calculator... I'm always using my own (http://raschedv.net/tool/bpmcalc.html), it's a handy Javascript tool.
- Type in '4' beats, '1' second and '936' milliseconds.
- Press [Calc] button behind bpm.
- It will tell you: 123.967 bpm.
Hmmm... do we really believe the song has 123.967 bpm? No! Obviously it has 124 bpm, because most modern music is built on an even bpm value. Ok, let's check it:
- Type in '124' bpm, and press TAB to confirm (now it will show 124.000 bpm).
- Press [Calc] button behind sec.ms.
- It will tell you: 1.935 sec.ms.
Seems like our selection was about 1 ms too wide. Anyway, we know it has 124 bpm, so....

9. Create a new track, let it be Mono.
10. In this new Mono track: create a Click Track, let it be 124 bpm, 4 beats per bar, 2 bars. (this is quickly done by filling out Audacity's formular)
11. Drag the Click Track horizontally (see the left-right-dart in Audacity's toolbar?) until the first bar is positioned below our selected drumloop-bar.
12. Zoom in at our starting position. Make sure that the starting Click begins exactly at our starting position (while zooming in until you see the discrete sample steps, you may have to drag the Click Track to avoid getting it out of sight).
After having positioned the Click Track's beginning exactly, you can zoom out again, until our selection end is visible.
13. Zoom in at our selection's end. The deeper you zoom in, the better you will see that our selection border is not exactly above the Click's start, in fact it is a bit too far on the right. Well, we knew our selection was about 1 ms too wide.
14. Ok, being on a low zoom level, where discrete steps are visible, we can adjust our selection in dragging the end to the left, until it is placed exactly above the Click's waveform's beginning.
15. Again playback your selection in Loop Mode (Shift+Play) and have a last check.

Lean back and take a deep breath, you did a great job.
You not only can now export your (sample-step-accurately cut) drumloop - don't forget to add '124bpm' to the filename - furthermore you precisely fractioned the whole song (in case you extend your Click Track by another 50 bars or so). What does that mean? Well, due to your exactly positioned Click Track, you are able to cut out each bar(s) you like to. Sometimes there's a few interesting bars in the middle (intermezzo) or at the end (outro) of a song, where solo instruments or drums appear.