Here are the requirements for a Captain's License.
Minimum age - 18 years
Proof of Age, Citizenship, and Social security number (Notarized copies acceptable)
First Aid and CPR Training - Original or notarized copy of first aid and CPR course certificates. First aid certificate must be completed within the past 12 months from (a)The American Red Cross Standard First Aid and Emergency Care or Multi-media Standard First Aid Course, or (b) A Coast Guard approved first aid training course. CPR certificate must be currently valid from The American National Red Cross, The American Heart Association, or
Coast Guard approved or accepted CPR training course.
Completed USCG information and application form is available via http://www.uscg.mil/d1/units/msobos
Drug Test - USCG "Drug Free" testing requirements are listed along with the required drug test form at www.uscg.mil/d1/units/msobos
Physical Examination Report - USCG form is included in your USCG application form, and must be completed within the past 12 months by a Physician, Physician's Assistant, or a Nurse Practitioner.
Three Notarized Character Reference Letters - Recommendation letters from 3 different persons who have knowledge of applicants suitability for duty.
Sea Service Documentation - the applicant's sea service must be documented by each vessel's owner. If applicant documents service on his/her own vessel, some form of proof of ownership must be included. Experience gained before age 13 will not be accepted. At least half of the required service must be obtained after the age of 16. One day of sea service consists of at least 6 hours underway. For all licenses, at least 90 days of sea service must be within the past 3 years.
If you have all that then I would suggest contacting a Sea School for help passing the test.