That would be number 1 mistake on the mistakes to avoid 
)
Unless you are a large company with lots of resources then you don’t want to create a specific mobile site as that means maintaining two separate sites and indeed these days most users want to access the real desktop site anyway.
If you do want to create a separate site for mobile then the simplest option is to have a link on each site that the visitor can click to go to the mobile site or desktop site and you can remember their preference with a cookie perhaps.
However, I would advice against creating a specific mobile site unless you have a very good reason and just create a responsive site instead that will suit all devices.