What do I need to take pictures indoors?

I'm kind of new to the photography world and teaching myself. I've always just taken outdoor photos using natural lighting. If I I notice sometimes when it is darker out my camera (Nikon D5100) won't even take a picture because it can't find a focus. If I wanted to take pics indoors like at a wedding reception what would I need in order for my camera to takes pics with no problem and not have trouble the whole time trryying to take one picture?

The first thing you need to do is add light to all those scenes.

Having the auto-focus assist light activated will also help the lens focus in low light.

The rule is that if the camera can't focus, then you need to add light and/or use a faster lens.

Your D5100 is fully capable of producing amazing photos, but you have to know some of the fundamentals of photography. Just having such a fine camera will not guarantee marketable images… Like those expected by the bride.

Many wedding and event photographers use an dedicated, external electronic flash (like the Nikon SB700) and a flash bracket when shooting indoors. The flash bracket will prevent redeye and that is important if you are shooting 200 plus images during the event

Well, a decent Flash Gun for a start. It will illuminate the scene but try not to use full on; it's best to bounce the flash from a ceiling or wall.
Also, you can increase the ISO to get more light into the scene. Try putting it to 800.
Lastly, if you are just using indoor lighting, then you must change the White Balance to Tungsten (or whatever lights are being used)

Fast lenses (aperture of f2.8 or wider) and/or light (speedlights are the most portable).

For flattering portraits, you may also need light modifiers (umbrella or softbox for example)