You can do a couple of things if you intend to use an SSD in addition to your regular drive.
Firstly, using Disk Utility is likely to be the easiest way to clone it, so long as you are sure that it will fit. Open Disk Utility, select your existing disk, and click the "restore" button in the top right. You will have something like the following screenshot, although in my instance I do not have any other disks.
In your own window, you will see your SSD disk in the left hand column. Drag it's name into the destination field. This will create a copy of the source disk, onto the destination disk. You may need to partition your SSD first - ensure it uses the same GUID partition scheme and options as your existing drive.
Next, prove you can boot from it. Hold the Option key down, and select the SSD to boot from. It should be identical, but way faster.
Now, you can choose how to take advantage of it, and your other disk. As you are already nearly full on your boot drive, you need to clear some stuff off it. A good choice for things that don't necessarily require speed are your iTunes library and iPhoto library, as well as any iMovie files. These are most people large file collections, and there isn't usually much requirement to have them accessed lightening fast.
Moving such data and folders around is one way to clear up a chunk of space to leave your SSD room to "breathe" without worrying about filling it, but it's a manual process that can be prone to error, as well as confusing for backups etc.
Another thing you can try is moving your entire home folder. Luckily, you don't even have to move it to test this option. Go into preferences, and locate your user. Right click on your name on the left column, and click advanced. You will see an option to store your home folder elsewhere. Click choose, and point it at your old disk. It will now boot from your SSD, but use the old drive for your home folder. This effectively moves all your data, but none of your apps off the SSD, which is a simple and efficient way to get a good balance between speed and capacity requirements. If you have large apps that don't need speed, you can move them into your home folders app folders to get them off the SSD too.
Once you have done this, and proved that it all works fine, you have some cleanup to do. You can either just wipe the old disk, then re-copy and delete (move, effectively) your home folder back into it, of you can manually just remove the hold folder on your SSD, and all the OS folders on the original disk expect your home folder.
There are millions of ways to skin this particular cat, this is just one (albeit the simplest one), and its what I intend to do as soon as my optibay arrives :)