You should for sure recognize a purine and know there are two types in nucleic acids (it's the double-ring structure and consists of A or G). You should also recognize pyrimidines and know which nucleotides fall into this category (single-ring with C and either T or U, depending on whether it's DNA or RNA).
Each nucleotide has the nitrogenous base attached to the 1' carbon of the pentose sugar. And you number the carbons sequentially from that starting point.
So here: I'll leave you with a challenge. Draw a pentose sugar attached to a purine (don't sweat the purine details too much... just make it distinguishable from a pyrimidine). Be able to alter your figure to depict a ribose, a deoxyribose, and that really strange entity used for Sanger sequencing: a dideoxyribose. If you can, depict where the phosphates fit on the structure, and then number the pentose carbons according to standard conventions.
Here's how I'd do it (quick'n'dirty 3 minute video):