To write-off business expenses on a sole proprietor LLC, those are reported on schedule C of your 1040 personal tax return.
https://turbotax.intuit.com/
For a partnership, you would use Form 1065.
For a S-corp, you would use Form 1120-S.
https://turbotax.intuit.com/small-business-taxes/
For a C-corp, you would use Form 1120.
https://turbotax.intuit.com/small-business-taxes/cd-download/
