I've just ordered one myself. I have lots of tips and tricks for the Holga 120 and Diana cameras on my website, and when I've had a chance to check out the Holga 135, I'll post some tips for that camera too:
http://www.dianacamera.com/
I'll also post some basic how-to videos for the camera on my channel at youtube:
http://youtube.com/kaituba
You can get a fisheye lens for the Holga 135, but I've only seen it on ebay. Look for the Holga Fisheye FEL 135.
The Holga 135 is a primitive 35mm film camera, but in the hands of a photographer with a few basic skills in composition, it can take some photos that are quite beautiful.
When using the camera, pay attention to what's in the frame. Don't let anything in the frame that distracts or pulls the eye away from your subject. Quite often, you can help improve a photo by merely moving over a few feet, or changing the angle you are facing. Take lots of photos, and don't expect every one to be a masterpiece. Think of it as trying to get your first 100 mistakes out of the way. Once you've done that, your photos will improve. When you get your photos back from the lab, take a look at them and see what's working and what's not working in the images. Then go out and put into practice what you've learned.
Be patient, it takes a little time to gather the skills it takes to become an excellent photographer.
Good luck!