How to make nikon d3100 focus correctly?

I have a nikon d3100 and I want to shoot a tutorial for youtube, and in my tutorial I bring certain objects very close to the camera to show detail about it, but its very blurry and all of the focus is on the back, where I'm. Is it possible to make my camera to keep focus whenever move closer or bring an object to the camera, etc?

Set your autofocus to continuous/subject tracking.

You are probably exceeding the minimum focusing distance of your camera. You can get extension tubes that go between your body and lens which will allow you to focus on smaller things.

I believe you would need to buy a better camera. Nikon is a "still frame" specialist to begin with. You will need a continuous AF and tracking which you will not find in a entry level camera. I think you are looking for the Canon 7D which prolly isn't what you want to buy just to make a youtube tutorial.