How do you set up a password for wireless internet?

MR.S: How do you set up a password for wireless internet?
I have wireless internet at home, I have DSL and use Linksys to have wireless internet but lately my internet is kind of slow. I think someone is using my internet. I see that others have password protected wi-fi internet so does anyone know how to set up password for wifi internet?

Answers and Views:

Answer by vern
You need to log on to your router. I hope you know UI/PWD, and make it secured by adding password. Anyone within range can now use your wirelss access.

Answer by tbshmkr
Web search for the make & model of Linksys Router
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Set up Password & MAC filters for start..