I got a D5100 and want to buy a LCD screen protector & UV ray protector, but how to know which model to get? I'll buy this from online, so how to find the right pair?
You don't need them, camera stores love to push those because they make a lot of money selling them but you really don't need them.
I got a nice mid-range UV filter with my camera, it took me a few months to understand just how bad it is and now it's somewhere under the piles of document and junk on my desk - a complete waste of money.
A cheap UI filter will hurt image quality, the most obvious (but not only) way is flare - any time there is a light source in the picture you will see flare, strange artifacts or just a reduced contrast affect (like a fine fog) over the entire image.
A mid-range UV filter will still reduce image quality (but less than the cheap filters) but cost a non-trivial amount of money, for that kind of money you can buy something that is actually helpful (a lens hood, a cheap external flash, extra batteries, etc.)
A high-end UV filter will reduce image quality by a negligible amount but will cost you as much or more than repairing a lens damaged front element - so there isn't really any point in getting one.
A screen protector at least is cheap and doesn't hurt you (unlike a UV filter) so there's no harm in getting one, but I just like to point out that the actual LCD is behind a tough plastic layer that is there to protect the screen (so you are actually getting a screen protector protector) and that a scratch on the screen (just like a scratch anywhere else on the camera body) doesn't effect the camera's ability to take good photos in anyway.
If you can get one really cheap and it makes you feel better (or safer) than get one - just get one the size of your camera's screen (just the screen, not the entire back) and you'll be fine. If you can't get one the exact size just get a bigger one and cut it to size.
Assuming that by "UV ray protector" you mean an UV filter for your lens, the short answer is: everything (that fits) goes.
Screen protectors are available both in appropriately sized and trim-it-yourself models, but ultimately all are a plastic layer.
You should buy an UV filter of the correct diameter (look under the lens for the number preceded by an ø symbol: that'll be the diameter of the filter thread in millimeters): it's possible to use a step-down ring to install a bigger filter, but unless you already know you're going to buy lenses with different filter ring diameters you shouldn't bother, especially since nowadays UV filters are only used to provide an "almost disposable" dust and wear protective layer and thus are generally left almost always installed.
Most cheap filters are uncoated, unlike the front element, and can thus increase glare in the wrong conditions: in my personal opinion the ~13 € price for the cheapest 67mm UV filters outweighs this problem.