Yes, you must reduce your education expenses by the GI Bill you recieved, but if you also paid for tuition, fees, books and/or supplies out-of-pocket, be sure to also enter those additional expenses.
After you enter your 1098-T continue through the education section and interviews to also enter the costs of books and supplies.
If you paid for some of the classes, Box 1 on your 1098-T should be larger than Box 5.
Compare your school account statement for 2018 with your 1098-T. If the 1098-T doesn't seem right, contact your school's financial aid office.