As before, "Life" (by Purves et al. I think) is a good place to start.
You might want to have a look in the library at the basic genetics books before you go out and buy one. Genes VIII will see you through your degree, but will be a bit complex to begin with.
Biochemistry by Stryer et al. is another good one for the basic stuff.
Genetics, unlike other bits of biochemistry, is very visual = it's difficult to visualise the Calvin cycle, but not DNA replication. It helps to find a book with lots of big drawings in and add lots of nice drawings to your notes. There's not one book I can really suggest, but if you want to do some background reading start on bacteria (E.coli).
Don't try any Eukaryote stuff before you understand bacteria.
As a start, if you can define transcription, translation, base pair, nucleotide, RNA, DNA, mRNA, rRNA, tRNA, ribosome, protein, DNA polymerase, RNA polymerase and genome and know what they all do you're well on your way and should be able to go out and find a book what explains things in ways you like. Trying to decode the books with little to no knowledge is quite difficult. Some books (molecular genetics of bacteria - Dale) have very simple line drawings which some people find useful, others (annoying also called "molecular genetics of bacteria" - synder and champness) have very detailed drawings. Depends what you like.