Nintendo does not officially have a program set up to give out free Wii points. Websites that claim to give out Wii points are not to be trusted.
Nintendo sometimes runs special promotions that can be found on its website. Otherwise, sites that claim to give out free Wii points are almost certainly scams, and should be avoided.
There are many scams available online dedicated to fooling users into giving out private information. The online term for this is called "Website Phishing". They do this by pretending to be a trusted person or company and asking for passwords, usernames, or other personal information. This results in stolen information or even full-on identity theft. Do not give out any personal information if there is any doubt about the source.