Soph, I think you're going into your first fair with the right attitude, treating it as a marketing exercise. You'll be making a lot of tweaks over time for future shows but use this one to get your name & products out there, see what sells & what doesn't, see what customers are interested in, see if you get any special requests or suggestions from customers, etc...
The information & experience that you'll gain from this first fair will serve you well as you continue doing more & more fairs. You'll know which products to make more of, which to possibly discontinue, you'll get ideas from other vendors displays that you can use to tweak your display, you'll discover if your price points are where they need to be & so on.
One big tip that I would recommend is that you try if at all possible to make friends & network with other vendors when you're at the fair. Believe me, sharing information with other vendors can be GOLD, especially if the other vendors share information about other fairs & festivals that they've done well at. That information alone will save you a lot of $, time & aggravation when it comes to finding more fairs to attend.
When we first started doing fairs, we jumped at every fair we could get into, lots of trial & error. Over time, we weeded out some bad shows & added some new shows. But our best & highest selling shows we found out about by speaking with other successful vendors. We now have a handful of other vendors who we share show information with & have compiled a list of over 30 additional shows that we didn't know about, until the other vendors recommended them to us.
Good Luck at your first fair. Be sure to report back after the fair & let us know how it went!