Johnny - negative marking is where you are penalised for an incorrect / blank answer on different values.
kuppusv - no, you do not need to attend a class. For many people it is better to just acquire some of the text books (Official Guide, Kaplan or similar) and learn the concepts and practice. This would be my advice, at least at first. After a while of study, you should be in a good place to judge yourself whether you would benefit from some instruction. Some people do and find it valuable, some people do and don't, many people never bother to pay for the courses available.
The negative marking question is tricky. You cannot leave a question blank in the GMAT - it must be answered to get the next question. You are penalised for unanswered questions at the end of the test (ie - the ones left at the end), but noone is sure quite how much this is.
Hope this helps to answer your queries.