Did you mean "uninstall" Lightroom 5.7? In either case, you don't need to install your old Lightroom, nor do you need to uninstall it. Lightroom CC will be a complete new installation.
i have lightroom 5.7 in my mac book pro , like to know if i buy the subscription $9.99per month of the creative cloud do i have to install my old lightroom 5.7
No, they are separate licence's. You can continue to use LR5.7 on the mac book pro, and install LR CC on up two additional computers, providing that the LR CC installations, are for your own use and are not used simultaneously.