New LLC and tax filing

If this is a single member LLC and no S-corp. election made, you will report all income and expenses on your personal return under Self Employment Income.  That means you will file taxes next year. You might want to take a look at estimated quarterly tax payments to see if you need to make any estimated payments to IRS and/or state. 

Estimated taxes | Internal Revenue Service (irs.gov)

If it is a multi member LLC, you will file form 1065 (Partnership return) and then report income your personal return from Schedule k1 from partnership.  

Just to clarify, since you indicate you are a single member LLC with income starting in September, you are required to pay estimated tax payments if you expect to owe tax of $1,000 or more when your return is filed. You would file a Schedule C which is included with your 1040 return and any withholdings via an employer will be included as payments with your return.  The total income earned on a W2 as well as your self employment earnings on a 1099 or otherwise via your LLC will also be included.  It would be advised to make the last two ES payments if you believe you will owe:

You can use the taxcaster to estimate your income and tax due.  Self employment income (LLC) is taxed at 15.3% for social security and Medicare on top of regular income tax for your tax bracket.  You can use this table for 2024:

cheers!