First of all - if you think you will fail - then you will! So what you need is confidence.
Check out the syllabus from the BHS site
http://www.bhs.org.uk/Examinations/bhs_stage_1_syllabus.htm
and go through each part and make sure you know it all thoroughly. That's for the care part. For the riding, again check what's on the syllabus and if you're not sure whether you're up to it - ask your instructor to go through it with you.
You could also ask your Instructor to give you a 'mock' riding exam and ask them to check some of your stable management things - like tail bandages, etc.
Study books specifically for the stage one exam like Hazel Reed's book which I have found to be excellent.
When you feel more confident, book the exam and on the day, take a deep breath, relax, try to enjoy it and remember - the other candidates are all just as nervous as you BUT the examiners really do want you to pass and they know you are nervous and they take that into account.
Anyway - hope this helps, good luck and let us know how you get on.
Evelyn.