First go to google.com and click the 3-by-3 grid that is circled in red below. 

Once you have clicked on the 3-by-3 grid, you need to select the "Gmail" icon that is circled in blue below.


Once you have selected the gmail click by the green line drawn below to enter your school e-mail. Once you have typed your email address, you can click the next button that is circled in purple below. Do the same for your password and you are ready to start using Gmail!