Step 1: Do not take the test in December if you are serious about a 600. It is highly unlikely that you will be able to increase your score 3-400 points in 3-5 weeks.
Step 2: Get some study materials before you start doing problems. You need to know how problems are constructed, as well as the basics of quant and SC. Then, buy a copy of the OG and start doing problems in timed blocks.
Step 3:Take a GMAT PREP test and see if you attain a 600+. If so, schedule a GMAT exam. If not, go back to Step 2 and relearn the basics.