I find the most difficulty people have with Chem is when they don't write out every part of an equation. It may seem silly to always annotate when you know what the variables are. However, if you always write everything out, you can't go wrong.
For example: If you're given the equation Na2O + 2 HCl ==> 2 NaCl + H20 and given the question if you have 18 grams of Na2O, how many grams of NaCl will result?
You need to write down for every mol of Na2O, you have 2 mols NaCl (you'll need this ratio later). Then set up your dimensional analysis, making sure you annotate every compound & unit:
18 g Na2O x (1 mol Na2O/62 g Na2O) x (2 mols NaCl/1 mol Na2O) x (58 g NaCl/1 mol NaCl) = 34 g NaCl
By writing down all your units & compounds, you can cancel g Na2O, mol Na2O and mol NaCl to end up with g NaCl.
While this is just a simple example, it really comes in handy when you move in to more complicated equations (e.g. kinetic theory, ideal gas law, etc.) For example, the constant R that is used in the ideal gas law is 0.08206 (atm*L/mol*K). If you write down the units, it's easy to make sure you cancel the proper units to arrive at your answer.
It was stated above, but I'll say it again: Khan Academy is a GREAT site for all things science!