It's best to put them in a crate when you can because the diaper can cause an infection if she wears it for too long.
Just to clarify, you take the diaper off when you put them in the crate.
Age of first heat varies, from 6 months to 24 months. Mine have been 21 months, 17 months, 17 months, and 15 months. I always figure 28 days just to be safe - and they may all come in together, as one will pull the others into season.
Never underestimate the lengths to which a dog will go to get to a bitch in season (or vice-versa) - take more precautions than you think are necessary just to be sure! In my house we have a two-door-minimum rule between bitches in season and male dogs (crates count as one door); dogs can and do mate through crates, fences, etc., so be extra-vigilant.
The amount of bleeding will vary, by bitch and with each season. Boxers in general are clean dogs, so most bitches will keep themselves pretty clean, but they can only do so much. Usually the vulva will swell noticeably - though with Emma's first season I really never would have known if I hadn't seen a drop of blood on the sidewalk, she barely swelled and bleeding was minimal.