Welcome to Nico!
The wheel speed sensors are mounted on the back of the wheel hub. So, if you look behind the wheels they'll be near the steering linkage on the front and near the brake on the rears. It'll be a black cylindrical object about a half inch in diameter and a half inch long with a thick black wire coming out of it.
This picture also gives you a peek at it after the hub's removed. It's that little bugger poking out just to the right of the silver shaft.
I don't know what you need to do to get at the rears.
Be warned though, if you're trying to replace them they won't just pull out because they'll be seized in place. On the front you'll need to remove the wheel hub like you're repacking the bearings and use a small socket or something to put over the sensor and hammer it out. Some members have had luck using a torch to heat up the area before hammering. Unbolt them and try just yanking on it first; you may get really lucky!
rgk gives you the lowdown on
O2 sensor location here.