What software comes with the $19.99 subscription?
I currently own a past version of LR on my desktop will I have to uninstall that if LR is included
If you're talking about the Creative Cloud Photography Plan, a quick look at Adobe's page about it will tell you what's included: Lightroom and Photoshop | Adobe Creative Cloud Photography plan
(Photoshop and Lightroom)
Secondly, you wouldn't have to uninstall your old version. You can have multiple versions installed.