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AtomicWoman
1,747 Posts
Don't pray -- make flashcards!
OK, pray AND make flashcards! 
Remember you first need to know the DOMAIN geometry and then you can figure out the MOLECULAR geometry. So a molecule with a tetrahedral domain geometry and no lone pairs of electrons will also have a tetrahedral MOLECULAR geometry, but a molecule with a tetrahedral domain geometry and one lone pair of electrons will have something different for the molecular geometry -- in this case, it will be trigonal pyramidal.
So make yourself some flashcards, either with drawings or words. I made word flashcards that said something like this: 4 Domains [around the central atom], 1 Lone Pair: Domain Geometry = Tetrahedral Molecular Geometry = Trigonal pyramidal.
Here's a website with some images (if you are more visually oriented) and quizzes (I am this guy's biggest fan):
http://chemistry.alanearhart.org/Tutorials/Shape/shape-part1.html
Just keep doing it; after a while you'll recognize patterns.
As for polarity, there are a couple of simple rules (it's a bit more complicated than this, but this is usually enough to know for chem I):
If there are 2, 3, 4, 5 or 6 (linear, trigonal planar, tetrahedral, trigonal bipyramidal or octahedral) atoms around the central atom AND all of the atoms surrounding the central atom are the same, then the molecule is NONPOLAR. All the dipoles cancel out.
If the domain geometry is trigonal bipyramidal, but the molecular geometry is LINEAR, then it's also non-polar.
If the domain geometry is octahedral, but the molecular geometry is SQUARE PLANAR, then it's also non-polar.
Again, make yourself some flashcards. Mutter this stuff while you'll driving. Tell your SO over dinner.
Pretty soon it will be comfortable for you.
One trick profs often do is give you a molecule like CH3Cl. When you draw it, it is tetrahedral and you're tempted to say it's nonpolar. BUT notice that only 3 atoms around the C are the same; the 4th atom is a chlorine. Therefore, the dipoles do NOT cancel out and the molecular is POLAR.
Always, always, always draw the Lewis structure. That will help you quickly realize that CH3Cl is polar. Also, you'll learn to recognize patterns. As soon as you see an O surrounded by 2 lone pairs, it will start to look like the pattern in the water molecule and you'll know it's polar because it's angular (or bent, depending on your prof's lingo). (This will work for other atoms, such as sulfur, too. That's one that you often see.
Finally, remember that strings of hydrocarbons are always considered nonpolar. Always. So whether it's C3H3 or C20H20, or any other combination of Cs and Hs, hydrocarbon strings are always nonpolar. (The difference in their electronegativities is not 0, but it is small.) Again, draw a few of them and you'll see why.
Good luck!