Celestia is a powerful piece of software that features a complete working model of our solar system as well as many major stars. If you really want to get into it you can modify the standard installation with much more high resolution textures, more detail about satellites, probes and other objects, or even simulations of fictional events from the movies!
To start with though, you probably just want to have a look around. The program uses several shortcut keys to make navigation easier, but the main ones you'll probably use are:
You can navigate around by using the Solar System Browser. The menu at the top of the screen (auto hides and unhides in full-screen mode) has an option called Navigation. From the drop-down menu, select Solar System Browser and you can choose objects to visit. Don't forget to click "Goto" or use the G key to go to them.
You can also change the options about what the simulation shows. You can display or hide things like orbits, galaxies, clouds, etc. From the menu bar at the top of the screen select Render and then View Options. Play around with this feature to see what happens.
You can also download scripts that do automated tours and I've written two so far which you may freely copy. Either download and save these files to your Celestia directory (default location is C:\Program files\Celestia\) or open them directly from this website and Celestia should recognise them and run them automatically.