If the LLC had any income or expenses in 2018, then you should file a Schedule C for it as part of your 2018 Form 1040. If you have already filed without including the Schedule C, you can file an amended return on Form 1040X to correct your original return.
Closing an LLC is governed by state law. You should check with the business bureau of your state's Secretary of State office for specific forms and procedures. At a minimum, you will need to pay any outstanding bills, claims for wages, and debts.