What are some things that I (or anyone) should take into consideration before participating in a tahara, the ritual preparation of a body for burial?
This is a huge mitzvah, so yashar koach for participating!
Certain people are not supposed to participate in this mitzvah. Ask your Rabbi to make sure you aren't one of them.
Some things to consider:
You will be dealing with a lot of water, some of which will be spilling onto the floor. Consider wearing galoshes or boots, or at least an old pair of shoes that you don't care too much about.
You may get splashed by water that is coming off of a dead body. Most chevra kadishas will provide some kind of apron or protection, but you're better off wearing clothing that you don't mind if it gets soiled. I was also told that some poskim consider it part of kavod hameis (honoring the deceased) to be prepared to get dirty in the process.
You will probably be standing for your entire time there. If you are quick, it may take you 40 minutes. If you're slow, it could be over an hour. If you end up with a double-header (two deceased for the same shift), you could be there for over 2 hours. So make sure you wear comfortable shoes.
In addition to standing for a long time, you will also be lifting the deceased and holding his/her body up for various reasons. And taharas are done at night, at the end of a long day. Make sure you are well-rested.
Since this is your first time, you may want to be prepared for dealing with possible squeamish-ness. Consider asking to be put in a group that is large enough that you could step out if you need to. Also, make sure they know it is your first time, so that they can avoid assigning you to a difficult tahara (some taharas are physically more challenging, and some emotionally).
Talking around the deceased is kept to a minimum. Ask the individual in charge of the team (or anyone experienced) to give you a rundown of what goes on, so that you don't need to do as much talking in the room (although this talking would be permitted).
When you get home, you will need to wash your hands outside before you go into your house. In the event you are doing a late tahara, make sure this is arranged before hand so that you don't have to wake anyone up when you come home.
Don't assume that you will be able to go home and continue business as usual, or go straight to sleep - you may need some time to emotionally unwind.