The first thing you need to do is set up an account. It's quick and easy. I do recommend going to the account settings and change your room number to something easy to remember, such as your last name.
Then, you are ready to have students complete Single Question Activities. In these, you do not need to write up questions or answers. You will have a question in your powerpoint or just read it aloud to your students. They will then touch the correct answer choice on their device, and you will have a pie graph of student answers.
If you want to type in questions so students read the questions and answers on their devices, you would need to create a quiz first in manage quizzes, then type them up. Then you would choose a Quiz Based Activity. The results will be emailed to you or saved directly to your computer.
Socrative is quick and easy to use. There are teacher and student iOS apps, or you can just use the mobile version, or even the regular internet version on your computer. I have just learned about infuselearning.com. It is similar to socrative, but allows students to draw as well. I haven't tried it in the classroom, but hope to soon.
Here is a Socrative intro/how to video:
Update: See my post about using socrative as an exam review.