i have a toshiba satalite A 305-s

Ram and upgrade hard drive to an ssd drive will net you the most amount of a performance gain.

Some laptops have soldered in CPUs so an upgrade is not even possible. Some are socketed (removable) but you still have to find the select few that your motherboard will support, and go through the process of dissasembling the entire laptop, not breaking any of the small delicate connectors, removing cpu, applying new paste and reassembly. It is not a task for a novice.

If you need a little boost then the ram and ssd upgrade will do the trick; if your goal is to make your 2007/2008 core2duo hold its own agianst a 2012 i3 then not gonna happen.