Nikon D3100 manual guide?

I just bought a Nikon D3100, and I'm a beginner. I want to take pictures in my friends party with is inside the house. But the Auto button is not that effective. Can anyone tell me how can i use the manual feature, and what ISO and etc i should choose? Thank you.
and also is it ok if i clean the mirror inside the camera?

You are going to need to use the flash. Inside there's less light.

Full auto should have popped up the flash for you! But even then it also has to raise the ISO [sensitivity]

Mmm it sounds like you should stick to automatic mode for a while. I don't know when your friends party is but if it is within 2 weeks.forget it I don't expect that you learn how all these camera basics work in such time.

In short, ISO/ASA is the sensitivity of the film or sensor. How higher how shorter the exposure time needed but.well noise shows up. Still todays DSRL" s are really good with noise.

Exposure time, how long the mirror is flipped up and the sensor is taking an exposure

Aperture, how small the hole is that the variable blades in a lens are set at. How smaller the whole how longer the exposure but also how more is sharp.

These are the basic ingredients.

My problem is your really your problem well you know what I give you a few tips. Put the dial in P mode, keep the pop up flash up and keep ISO on auto. Or keep it even in full auto.

Because learning manuals modes takes some time and I don't feel that if within 2 weeks you have a a birthday to shoot you should play "the photographer" instead be a "point and shooter"

Then since you own a DSLR look for a local cheap course in photography. A simple course that will inspire you, maybe a nice and simple book with enough of pictures.always useful as cameras have so many buttons.

Learning your camera, when it is a DSLR, is it can take a year before you really feel in control of it. So don't worry AUTO modes. Will be fine. And take your time to learn the basics.read the books. Play with your camera

BUT REMEMBER. Trying to learn your camera ON THE SHOT. Doesn't works. So your friends birthday.shoot auto! Give yourself a year to get used to the idea of control, of what means what. What if I put the aperture at F11. What then? It takes time to learn these things.