Don't despair! Take your time, relax and swing smoothly. That's why it's important to warm-up first (golf clubs). The fitter will be used to golfers being a little nervous and their experience will enable them to see past the poor swings and focus on the ones that more accurately portray their client's normal swing.
Do I have to buy new clubs or can my current ones be adjusted?
This depends largely on how suitable your current clubs are for you. A fitter may recommend a small change such as an adjustment of their lie angle to more flat, neutral or more upright in which case your current clubs will usually be adjusted for a small charge(Taylormade R7 CGB Max Irons). If a more radical change is recommended, this can include new shafts, grips or new clubs altogether.
Can I try before buying?
Some fitters will loan demo clubs for you to try as they want you to experience the changes on your regular course, under normal playing conditions and using your preferred ball (Titleist Pro V1 Ball)- as opposed to harder ones on the range. But this isn't always possible. They tend not to stock too many demo clubs required to cover every variant. You are putting yourself in the hands of the fitter and trusting their skill and judgment.
What if I can't get on with the clubs he fits for me?
A good club fitter won't be happy until you're happy! They will tweak and fine-tune them until the golf clubs are working for you. Once properly fitted you will look back and wonder why you ever struggled with clubs that were unsuited to you. You will gain confidence, play better, score better, and lower your handicap.