One of the simplest ways is to find some points on the graph.

In your case, you get the points (0,0), (1,2), (2,4), etc.

Now, you draw a line on those points.

Now, we consider the restriction. X should be greater than 0. You erase any part of the graph where x is negative and put a small circle (like in plotting a coordinate) around the point where x = 0. This will mean that the graph starts at zero, exclusive and continues upwards on the line y = 2x.

Here's an example:

http://p1cture.me/images/28124199657601053843.gif