Number one thing is discussing the equity split and what happens if one of you quits.
I have a friend who helped start someone selling a breakfast product. The plan was my friend would invest $20k and the partner would run it as a local business, and they'd split 50-50. Very quickly my friend got excited by the potential and started working full time to help, ends up investing hundreds of thousands of dollars, doing all e-commerce, marketing, negotiations, paying for industrial quality kitchen, etc, while the partner just mixed and packaged. Sales shot up and the company begins to have significant value, but still requires more investment.
They needed to switch from an LLC to a C corp so they could get funding and start giving stock options to attract/keep good employees. The partner conceded that my friend was doing the lions share of the work and had put a ton of money in while he had put in nothing, and that my friend should have most of the initial stock in the C corp. But they struggled to agree on an exact split.
So then the partner talks to friends and family who of course tell him "it was YOUR IDEA, you shouldn't have to give up anything!" and comes back refusing to sign anything (which scotches the C corp conversion). Eventually he's able to force my friend to buy him out for a totally unreasonable price.
So my specific advice is.
1) Never start a new venture as an LLC unless you don't plan to ever share equity with employees AND you expect it to be a cash flow business that will pay out all profits directly to partners.
2) Always force all partners to vest their equity over years. You can do incentive stock options, or restrict stock units, or however you want. If you can't afford a lawyer to draw it up yet, at least put the arrangement in writing and agree to it, so that if you have disagreements and need to part ways, you aren't being forced to buy half the business from some guy who mixed ingredients for 6 months.