Chances are, if you're getting the internal storage low space notification, it means that your app storage area is getting low. That's relatively easy to do since Motorola partitions off around 2GB for it. You'll want to move some of the data for your apps to the internal storage that's partitioned off for personal use (the "media area"). Go to settings -> Applications -> Manage applications. Then hit the menu button, followed by sort by size. Tap on an app, and for most you'll see an option that says "Move to media area." Do this for several in order to free up some space. This moves the data to a different area of the internal storage, not your SD card. But it serves the same function as moving it to the SD card. It's due to the way Motorola partitions the internal memory (2GB for apps, 8GB for personal media storage, and around 6GB for system files).
If you have personal files (documents, pictures, videos) in the media area that you want to move to the SD card, use a file manager like
Astro to navigate to the files, and then copy/paste them over to your SD card. It's just like moving files on a computer.
Similarly, transferring files to your SD card from your computer is just like using a flash drive. Hook up your phone to a computer with a micro USB cable, make sure USB Mass Storage is selected from the notification bar, and then your SD card will show up as a removable disk. The Media area of the internal storage also shows up as a removable disk named MOT. Then copy and paste the files from your computer to the SD card or the internal storage. Just like a flash drive.