Photoshop has a wide array of tools the graphics artist can use to create images. In particular, there are several different ways of creating lines in Photoshop. Which one you choose to use will depend on the project and the type of line you need. In each case, there is a way of adjusting the width or thickness of the line being creating.
Select the "Brush" tool. In the options at the top select the brush size you want. Now any line you draw will be the same width as the brush. The "Pencil" tool can be adjusted in the same way.
Select the "Rectangular" shape tool and set the options at the top to "Fill." Use the tool to draw a shape on the canvas. Now go to "Edit" and select "Stroke." In the dialog that opens set the width for the line.
Select the "Pen" tool from the toolbar. Click on the canvas twice to create a line path. Click on the "Path" tab and right-click on the listed path. From the options select "Stroke Path." In the dialog that opens you can choose either the "Brush" or "Pencil" to apply the stroke in which case it will be the same thickness you set in step 1. However, you can also click on "Simulate Pressure," which will cause the thickness of the line to vary as it is applied.
Select "File" and click "Open." Browse to and open a small image you would like to apply a line to. You will notice that you are limited in the degree to which you can adjust the line width, because of the small size. To solve this select "Image" and click "Image Size." In the dialog increase the size and resolution of the image. This will make the methods in the previous steps work better.