Place the slide on the stage, and focus on it. I hear they usually accept bribes, such as chocolate. If your lab is really disorganised/poor/prone to breaking stuff, a local core imaging facility should have one somewhere. So go ahead and dig one out of some dusty drawer in the corner of the microscopy room. Usually they come as a mm long thing that's divided 10x into 100um units, which are in turn subdivided again into 10um ones.
So what you need first is some sort of micrometer slide (microscope slide with a scale on it). Probably about time I make some sort of reference post.
I've been asked a few times to show how to calibrate the scale if it's not included in the image metadata. I put the rest of the post under the fold as most of you probably don't care. I'm by no means anywhere near an expert with ImageJ (and don't do anything sophisticated, like macros or even much 3D/4D work), but there's a few basic things that I had to learn, and would like to share with anyone interested.
Also, the customiseable shortcuts are a blessing (and a curse for one's muscoloskeletal system, especially in the wrist region). For those not in the loop, ImageJ is a popular open source image processing program for microscopy due to the availability of some rather sophisticated plugins, it can be a very powerful tool in the right hands. Due to popular demand, I'll sporadically write up a series of posts on some ImageJ basics.