First you need to decide whether you want a compound microscope (only useful for viewing slides) or a stereo microscope (can view whole objects). The received wisdom is that a stereo is much more useful for beginners, and I agree: we have both, but get much more use out of the stereo. (It doesn't have such high magnification, but you can use it for viewing slides too, so it is more versatile, and in practice, the "let's look at it under the microscope!" moments happen with whole objects.
Then you need a reasonable quality instrument without spending more than you want... General advice is to avoid toys and look at things marketed to schools. A good quality supplier is useful - in the UK, Brunel Microscopes,
http://www.brunelmicroscopes.co.uk/is the obvious one. There's a yahoo group whose name I forget but it was pretty obvious, whose members were useful, but it's clear there are pseudo-religious wars to step into the middle of, e.g. some people feel it's always better to buy a second-hand microscope rather than a new one at the same price, but that requires expertise that many of us don't have... My first microscope came from ebay and has been fine, but that can be dangerous. What you want is an expert, which I am not...
