As far as I know, a ruler will be enough for measuring the length (if your penis is quite straight).
You measure from ABOVE the penis base (the side that you see looking down, not from the sides nor the balls' side).
You should press the ruler into the skin so the fats won't take place into counting (lowering the result by starting higher).
Not knowing your age tho - you should know some things:
First and foremost - "size doesn't matter" isn't a cliché. As you grow up and get experience, advices, and most importantly - learn how to listen to your lover, and get to know him - you'll learn that size isn't the most important thing and bigger isn't always better.
Secondly, Most men out there are usually lying or just not measuring right, resolving bigger numbers. Don't ever be ashamed of your body, and don't let anyone make you feel bad for what you were born with.
Best regards,
Simba