Fields are full of stickers and prickly pear. You've been warned. Mocs are the best, whether they're Dyers or homemade. I've seen few from Tandy that will hold up to a week at rendezvous, but many people start with these. Rubber soled mocs are a no-no, but if you're a newbie, and keep both feet on the ground all the time, most folks won't say anything to you.
Cowboy boots are definitely out.
Round-toed boots such as Wellingtons are acceptable as long as there is no decorative stitching. I am partial to the SW style mocs offered by a couple of firms that have a thick rawhide sole. The cactus can't get through it, and neither can most of the rocks. No sandals, tennis shoes, or other modern stuff even for kids. Solid color wool or cotton socks are appropriate.