Tacking can take forever, so get there early if you want a lesson in between. I'd give myself at least an hour in the beginning because becoming familiar with the process AND the horse takes time. Be quiet in your horses presence and by that I don't mean silence if you're a talker (I'm a talker). What I'm saying is be calm, and in control of yourself. That was the best advice anyone gave me (and I got it here, btw). Give your horse time to get to know you and your scent. And give yourself time to adjust to being in a horse's presence. They're big animals, of heart and of spirit. But they're also prey, so they're alert to you.
Assuming you've been on a horse before, if you have a few minutes after grooming and tacking (smiling here), you'll learn how to mount, to sit properly and hold your body, how to keep slack in your reins, position your arms, your legs/feet. THEN you'll learn how to get your horse moving. Or how to tell your horse to hold still. Some people don't have a clue how to tell a horse "okay, lets go".
When moving you'll learn to stretch yourself while allowing your horse to warm up. A good warm up is important for your horse and for you, so the two of you will slowly walk around and around and around and around the arena and while you do, you can stretch your arms, your legs and become familiar with the movement of your horse and how you balance yourself.
Take it slow. No hurry. Everything comes in time.