When buying lenses for Nikon D5100?

Please forgive my ignorance, but I don't know anybody who is into photography. When buying a new lens, how do know if it is going to fit my camera. Do all Fmount lenses fit and how will I know if the lens has a built in motor?

Sorry for asking such a dumb question.
Thanks!

Added (1). Thanks for the info, but you forgot to tell me if any Fmount will work.

Articulating screen on the Nikon d3100?

I recently bought a Nikon D3100 as my first dslr and my friend told me that it had an articulating screen. It turns out it doesn't at least on mine. I called radioshack the other day and they said that the stores model had an articulating screen. Now my question is:

Is there a kit or something that i could buy to get an articulating screen on my d3100? Is there any way for me to at least get a work around for it? I need to be able to keep the camera low to the ground but able to see the screen while i'm standing.

Added (1). I actually after about 45 mins of searching i found this: http://www.coollcd.com/619ah-7-inch-tftlcd-vga-monitor-no-touchscreen-hdmi-new_p634.html

i will be using my d3100 in a fig rig and i could possibly mount this on it? Would that work?

Are Nikon or Canon entry level DSLR's better for video?

I'm on the fence between a Nikon D3100 and the Canon T1i. Snapsort has given them a tied review:

http://snapsort.com/...ikon_D3100

Both seem find to me by looking at sample videos. Nikon is cheaper as well. But I often hear the old "Canon for video, Nikon for photos". Are there real strength differences?

I wanna buy a new camera lense?

I have a Nikon d3100. And right now i have 18-55mm, but i want to get a better lense. One that is affordable and worth the money. And suggestions?

Added (1). I want the new lense for taking shots of friends and family and sport events.i've been told the 55mm-300mm vr was good?

Should I purchase the Nikon D5100 now or wait?

Should I purchase the Nikon D5100 now or will Nikon come out with an upgrade?

Nikon d3000 will focus in and out but won't snap picture?

It happens randomly.sometimes itll work just fine and others it will focus in and then out and not take the picture.ill switch to manual focus and itll be fine but i don't want to manual focus all the time. I take off the lens and wipe off the connecting parts and it doesn't do anything. Its very new, and i don't use it much.i've never dropped it. I have no idea what it can be.switching settings doesn't hep either.

Help choosing an underwater point and shoot camera?

I'm in the market for a waterproof point and shoot camera, and was wondering if any of you own one and liked it. Please don't just give me a link to Amazon of a camera you googled, I want an answer from someone who has one. And I'd like a point and shoot. I have a Nikon D7000 and lenses, I don't need a DSLR. Sorry if I sound rude AT ALL, I'm just trying to help narrow down my criteria!

How to use the 1.8 setting on the 50mm f/1.8?

I own a nikon d90 and 50mm f/1.8 lens. Whenever I change the setting on my lens to 1.8, something comes up saying FEE. How do I program my camera so I can shoot photos using the 1.8 setting?

My Nikon MB-D80 is stuck on my camera and won't come off?

To i resived a Nikon MB-D80 from amazon…
it was all packed no beaks or anything its full is a Nikon
so i put it on my Nikon D90 and fantastic it works great.
i must of took 50 shots in about 2 hours then whent to remove the battery and the pin that on the lock that opens the battery door fell out, problem.
so whent to remove the hole Grip… Won't come off
the wheel is really stiff. Won't spin… Then spins freelly and goes stiff then ships freelly. But won't come off?
i only got it today? What do i do?
please help i don't won't to have to send the hole thing off and not have it for mouths and the closest Nikon reper center it 68miles away about 2 hours drive in london!

what do i do?

I'm looking to get a new nikon DSLR?

I've been interested in photography for some time and have had the nikon d80, but it doesn't do all that i want/need. ANy suggestions for dslrs preferably under 1000$? I like to shoot landscapes and am looking into filming too.