Under Control Panel, chose "Performance and Matainance"
and then "System". Unless you switch to classic view,
then it is just "Control Panel" then "System".
You apparently want to minimize the amount of memory Windows uses, but that's a counterproductive desire. Windows is designed to use all, or nearly all, of your memory, all the time, and that's good not bad. Unused memory is wasted memory. You paid for it all and shouldn't want to see any of it wasted.
Windows works hard to find a use for all the memory you have all the time. For example if your apps don't need some of it, it will use that part for caching, then give it back when your apps later need it.
