How do I set up a username and pword for my wireless internet connection?

I have a felling that someone is using my wireless internet connection because it has been going very slow lately and I want to know how to set up a username and pword for the internet connection. I have a "Blanc" (name of company) wireless router. If anyone that has the same router or someone that knows allot about computers, tell me how to set up a username and pword??