I recommend the nolo books. If you are in the USA, there's some legal guidance but even if you are not, it covers a lot of different aspects of how to set this up. There's one on partnerships.

One thing to think about: if you are co-owners, are you going to be co-CEOs too? If not, be very clear about what is a decision for the CEO and what is an owner decision. I've seen partnerships run aground on this exact issue.

it's as simple as downloading the forms off your department of state's website, putting each of you as a 50% managing member and sending the forms in. there might be a few more forms and a small filing fee.

don't use legalzoom or pay anyone to do it for you. it's too easy.

you should research 50/50 businesses. far more common and successful is a business with a controlling partner. i'd advise that if you feel like that should be you, come up with a rational list of reasons why and then basically communicate that you plan to be the chief decision maker so there's no misunderstanding. and after all, if you can't resolve who will be what, how are you going to manage a 50/50 partnership?

Start from here: https://www.greggpollack.com/only-fair-way-to-do-startup-equ...

Be sure to use a shotgun clause.