That depends... First you need to figure out exactly what kind of RAM it has. SDRAM, DDR SDRAM, DDR2 SDRAM, etc. Then you have to find out what the maximum amount of RAM your laptop will support. At 512MB, you might already be as high as you can go, but Toshiba should be able to tell you exactly.
After you know what kind of RAM your system takes, and how much you can put in, you have to figure out where the memory banks are on your laptop. Sometimes they're very easy to access, sometimes you have to rip half the laptop apart to get at them. All depends on that specific system's design. Toshiba should be able to help you with that as well.
If this is sounding like a bit much for you, you could pay for a service like Geek Squad to install it for you. They could find the right type of RAM from the store inventory, and then install it into your laptop. This will likely add at least an extra $100 to the cost however.
how to upgrade ram in my laptop. it's toshiba satellite intel (R) celeron(R)M processor 1.60ghz with 504MB OF RAM