How do i?

You get your Mylot reputation rating (star) when you reach 100 points. Mylot sent an alert to all members regarding this. Make sure when you get an alert you read them. They are important. Here is a link to the announcement from mylot. http://www.mylot.com/w/discussions/573319.aspx

It's largely getting ratings for your replies. The star refers to your "reputation" as a user. Like, I've been here a few weeks, written a lot of posts, and my star is that. The idea's participation, and contributing to a discussion. You can rate other people's posts, and be rated, on what you post.

Just by posting more quality responds and the other people will rate you.

i still don't have one maybe if you'll get the best response tag they will give you a star

Mine apppeared when I hit 100 responses, I have no idea what the color or number means though. Something to do with the quality of your answers. You also can't paste into your answers until you hit 500 answers. SO just keep on answering.