How much is the higest exposer i can set in Nikon D3100?

Well i want to capture stare tails so can any one tell me how much to i need for that?
or do i need to buy a shutter remote?

You are asking how to capture star trails. For that, you will need to use a tripod and using the bulb setting, open the shutter and hold it open for at least 15 minutes to get a good base exposure

What you will be fighting is the ambient lighting in the scene vs the much lower light coming from the stars.

You will have to adjust the lens aperture to increase the amount of light the sensor records at those various apertures.

A remote will allow you to keep the shutter open without touching the camera, thus reducing chance of camera movement

This site explains how to do star trails: http://www.danheller.com/star-trails

You'll definitely need a good, sturdy tripod and a locking remote. You'll also need to know how to use your camera in Manual.