If you move the slide and what you're looking at doesn't move...it's not the slide.  Adjust the condenser, and it should disappear.  Spin the condenser up and down; everything you see that comes into focus and leaves as you move the condenser is just from a dirty condenser, lightbulb, etc.

If it doesn't vanish when you move the condenser, and it doesn't move when the slide does...check the eyepieces.  This is probably where it's at.  Spin each eyepiece around, and if the stuff moves in a circle, too, use lens paper and cleaner to CAREFULLY clean the gunk off the eyepiece.

Your scope can focus on stuff on the top and bottom of the slide, as well as the material in the center layer that you're SUPPOSED to see. Start off with a CLEANED slide. Try turning the coarse adjustment slowly with a 4X lens, from the very bottom position to the very top.  You'll see either two or three layers come into, then out, of focus.  The middle layer is actually the slide specimen.  The other two are dust on the top and bottom of the slide.

ALWAYS check in your atlas or the extra books so that you know what you're looking for.  Later on in the semester some of you might draw water fleas and think that they're green algae!  The slides are contaminated with other specimens--it's very hard to grow a pure culture of something, so there are weird organisms mixed in, plus assorted salt crystals and chunks of dye.  If you don't know what you're looking for before you look, chances are good you'll find something else!

The 4X lens will find almost everything.  It'll be small, but there.  If you're on the 100X lens, and can't find ANYTHING, and you've really moved the focus knobs from from their position when you used the 40X, it will save time if you give up on the 100X.  Go back to the 4 or 10X and a cleaned-off slide, and locate the specimen there first. THEN use the 40 and 100X.

If the 40X or 100X just will NOT go into focus, it's a good bet someone put 'em back dirty.  Take them off, CAREFULLY, and look at them.  If in doubt, use the lens paper and cleaner to remove the gunk that's keeping you from focusing.