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The essential principle behind star trails is stacking multiple photos together. To stack multiple photos first you need to understand how to take a single photo. It is all about the light falling on your camera sensor, since the light coming from the stars and dark sky is very low. So you have to get as much light as you can get without contrasting too much on the landscape involved, or getting movement of Earth in the photo.

So try increasing exposure time incrementally and see for yourself. In general cases, 15-17 seconds exposure will get you enough stars with the landscape involved (with wider aperture so that you can get enough light). So, in brief, this goes for normal star photography. For star trails all you need to do is stack those star photos.

Since you need to stack multiple photos together, you will need a sturdy tripod (unless you can stack photos perfectly in your photo editing software without ripping your hair off!). So grab your tripod and go ahead click multiple star photos and try overlapping those and see what you get.

For detailed technique, please follow this tutorial.