Open SQL management studio and connect to your SQL instance. Under object explorer, right click on the server name and choose properties - the 'Server Properties' dialog is shown
You can configure memory here, such as the minimum and maximum memory values allowed by SQL server
I think the defaults are min (0) and max (2147483647). This means SQL server will dynamically allocate itself memory when it is required.
http://technet.microsoft.com/en-us/library/ms181453.aspx[
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"Use this page to view or modify your server memory options. When Minimum server memory is set to 0 and Maximum server memory is set to 2147483647, SQL Server can take advantage of the optimum amount of memory at any given time, subject to how much memory the operating system and other applications are currently using. As the load on the computer and SQL Server changes, so does the memory allocated. You can further limit this dynamic memory allocation to the minimum and maximum values specified below."
The thing to note is, even though SQL Server has allocated itself memory, it will release it to the OS 'on demand' if other applications require it.
If you don't have very much memory installed on the computer (say you are running SQL Sever, Visual Studio etc) then simply set a maximum value in the server memory config. This will limit the performance of SQL Server though.
In short, don't worry about SQL Server, it's pretty good at managing memory - worry about your own code!