In the Tools menu, run the Auto-Configuration Wizard. It will ask you questions about what you want to do with OBS and test your internet speed, and then suggest basic settings suitable for your computer.
"4GB ddr3 , Core i5 2500, W 7 Ultimate 64 bit" is not enough information to make any suggestion. What settings are the best will be determined by what kind of applications you are trying to capture, whether you're trying to livestream or record, how fast your CPU is, how fast your storage devices are, how fast your Internet connection is, and what kind of tradeoffs you want to make (if you can't handle, for instance, both high resolution and high framerate-- what is more important?)