There are a couple of numbers you can try.
A lot of people like setting SQL Server to use 1 Gig less than the physical memory, so in your case 11 Gigs. This leaves 1 Gig for Windows and anything else running on the server. If you have a lot of very complex SSIS packages or are running other services then you will need to leave more RAM for them.
About a year ago Microsoft begun recommending that you set SQL to use 75% of the amount of memory in the server. So in your case this would be 9 Gigs.
What else will be installed on the server?
Ok i will try ,because there is only Sql Server, in your opinion if i set DB recovery mode set Bulk-logged,it is better then simple mode?for optimize performace?