There is one book in particular that might be of great help to you- for us it was a required purchase for our Pharm class.
It's called MedMath (here's a link so you can see it: http://www.amazon.com/exec/obidos/tg...glance&s=books
If your school doesn't require another book (there is another good one called "Math for Meds" that has new versions available), you can get a used copy from Amazon
or somewhere that sells used books for about five bucks. It's definitely worth the money, makes the math really simple, and includes all the conversions you'll ever need along with your formulas, etc. I love this book, I kept it after nursing school, and I still use it today to go over stuff that I might forget.
As far as the meantime goes, most of it is just memorization. Here's a site that you can look at that has basic conversions listed: http://www.nurse-center.com/studentnurse/nur11.html
Converting within the metric system is usually pretty easy- for instance, there are 1000 mg in 1 g. There are 1000 g in 1 kg. The same goes for ml. There are 1000 ml in one L. There are 1000 L in one KL. To convert these, you simply move the decimal points over- here's an example.
If you need to convert 1.724 g into mg, you move the decimal place over three spaces to the right (you go to the right three spaces because you are essentially multiplying 1.724 by 1000; each gram has 1000 milligrams in it- does that make sense?). So, your answer would be 1724 mg in 1.724 grams.
This is hard to explain in a post! Check out the links and let us know if you need more help.