Why does my Computer lag on Minecraft?

Hi Everyone, I have a fairly new computer and I am wondering why Minecraft is running laggy and slow. It is a HP Pavilion G7 and it has 4 GB of installed Memory . It has a Intel Core i3 CPU GHz processor and is supposed to have 2.53 GHz of processing power. I close everything when I play minecraft and I put everything on the lowest settings and it still lags like crazy! Can someone offer me some suggestions?