I'm assuming you are using a new version of C5.
1- You go to your dashboard
2- You look for the box that says MEMBERS and in that box you click on attributes
3- On the User attributes screen you have several attributes already defined, don't worry about those. What you want is your own attribute.
To make sure you understand let's just say an attribute is a question you want to ask and the attribute's value is the answer the registering user is giving that question.
4- At the bottom of the Attributes screen you have a list that says "Add attribute". There you can choose any type you want. Say you want to ask a simple yes no question (Do you do Sport?)
5- From the list you would select "checkbox" and click add.
6- You get on a new screen to set up your attribute
7- The handle has to be unique. You should avoid capital letters and replaces spaces with underscores. Something like do_you_do_sport or does_sport
8- The name you give the attribute is the question people will be asked: "Do you do Sport?"
9- Make sure "Field available in Dashboard User Search." is checked so you can search against this attribute later on
10- Make sure you check "Show on Registration Form." so people are asked the question when registering. If you need also check "Require on Registration Form." to make it compulsory

Now when you go to your register page, you will see the newly added question with its checkbox.

Now for the search part:
1- Go back to dashboard and in the MEMBERS block click on "Search users"
2- On the search users screen click on "advanced" at the top right hand
3- in what opens up click on "customize results"
4- In the list that appears check the box for your new question
5- Now when you show your users, there will be a column showing the answers to that question.

I hope this is helpful, don't hesitate to ask again if it's not totally clear.