Please help me with my grammar ?

Hi, I have been out of high school for a while. I have been a stay at home mom for 3 years; my brain has turned to mush! I have decided to go back to school. I took the comp test yesterday, I past the essay part & reading part. I failed the writing part were you are supposed to fix grammatical errors. I am awful at this! I keep putting commas in the wrong place, or putting too many. I went to the library & checked out a ged book, but I still keep messing up. My adviser said my scores do not make since & she wants me to retake the grammar part. Can any one give me some advice on commas & semicolons? That would be wonderful. Thanks in advance.