Here's a quick post about modifying Oracle Database System Global Area.
SGA is basically a certain amount of memory dedicated to an Oracle instance.
You can set the SGA size during the creation of the instance. If you need to increase it after the instance has been created you have to use some brief SQL commands
Connect to the instance using SQLPlus then run the following commands:
SQL>alter system set sga_max_size=YOUR_NEW_VALUE sid='*' scope=spfile;
SQL>alter system set sga_target=YOUR_NEW_VALUE sid='*' scope=spfile;
Where YOUR_NEW_VALUE is obviously the new amount of memory dedicated to SGA.