To form a Limited Liability Company, you can do it yourself or use an online filing service. If doing it yourself, simply visit your state's website and follow the procedure for filing an LLC.
Otherwise, use a professional online filing service that can complete and submit the paperwork needed to form your LLC in your state. Popular services include ZenBusiness and Inc Authority.
For more detailed information about forming an LLC, consider this book/ebook:
Form Your Own Limited Liability Company: Create An LLC in Any State