So Raspbian is installed on the SD card and the SD card is in the card slot, what's next?
Before booting the Raspberry Pi for the first time, make sure you've got a keyboard and mouse plugged in and connected the monitor to the HDMI port.
Once you are ready, connect the power via the micro USB socket and it will boot up.
When it's finished booting you'll see a screen like the following:
You can change some configuration settings here, most can probably stay as the defaults. One you may want to change is the boot_behaviour, so the computer will start the desktop on boot.
To do this, use the down arrow on the keyboard to highlight the option (boot_behaviour Start desktop on boot?) and press enter, then on the next screen select <Yes> this will take you back to the menu screen.
Again using the arrow keys, select <Finish> and press enter to restart the computer. This will then reboot and boot straight to the desktop.
The desktop will look like this:
Congratulations, you have installed and booted into Raspbian for the first time!