Do not look at the fireball and cloud. Under no circumstances should you look directly at the fireball. On a clear day, this can cause temporary blindness at very large distances.
...Avoid exposure to Gamma radiation. Try not to spend more than 5 minutes exposed. If you are in a rural area, try finding a cave, or a fallen log into which you can crawl. Otherwise just dig a trench to lie in, with stacked earth around you.