I would use a graphing tool like desmos or geogebra, or a texas instruments graphing calculator, there a number of tools out there. The first two options are free online tools. Using technology to graph is much preferred over doing it by hand.
If you must do it by hand, then the idea is to plug in various values of x to get corresponding values of y. For instance, if x = 7, then
y = sqrt(x-3)
y = sqrt(7-3)
y = sqrt(4)
y = 2
So the point (x,y) = (7,2) is on this curve. Repeat for other x values to get more points. Then plot the points all on the same xy grid. Lastly, draw a curve through those points. The more points you have, the more accurate the graph. Use graphing technology to help confirm you have the right graph.
As for the question of one-to-one, you are looking to see if the horizontal line test fails or not. After fully graphing y = sqrt(x-3), try to construct a single horizontal line that will pass through more than one point on the curve. You'll find that such a task is not possible in this case; therefore, this graph passes the horizontal line test and consequently the graph is one-to-one.
An example where a graph is not one-to-one would be a parabola. It is possible to pass a single horizontal line through more than one point on a parabola.