It is best to check with the company you are flying with but the following guidelines are a great place to start:

In general, dress like you were going for a nature hike for a couple hours.

Wear closed full shoes, like sneakers or boots. Do not wear sandals, flip-flop, crocks, etc. The balloon may land in rough terrain and you want to protect your feet.

If the season is cool, dress in layers so you can add or remove clothing to insure your comfort. It is not colder in the balloon than on the ground.

Wear long pants even in summer. This helps protect your legs in case the balloon lands in an area with unfriendly vegetation like stickers, thistles, etc.