Most doctors will start with the lowest available dose (I think usually about 150 mg) twice a day to start and watch for side effects. It needs to be taken with food or an antacid. The next increment is 200 mg. For MC it seems that most patients end up with 200 mg every 8 hours. Some go on up to higher doses up to 900 mg total per day if they become too tolerant of the dose over time.
You can ask your pharmacist for a product insert or go to TEVA's website to download a copy. That way you will have a better idea what to watch for. Some people even take a "pulsed" dose and just use it occasionally when they know they will be in situations where they are more likely to be stiff, but in general I don't think that is recommended. You would have to ask your doctor.
Hey Heather I am 18 now and I have been taking Mexiletine for 14 years and I cannot move without the drug. I was the youngest child in Canada for this drug to be tested on for myotonia and have experienced no side effects but have steadily increased my dose since I started. I am currently taking 1000mg per day because I am very active. You are going to be thrilled with your new mobility, however you will certainly notice when the dose wears off. Also becareful of increasing the dose too quickly or you may experience some side effects.