You have to be careful on selecting your co-founder. I know it's an already cliché to mention it, but selecting one is like selecting a spouse to marry.
You need to know the person first, and even some investors and accelerators dismiss automatically startups because the founders didn't meet before working together in the startup.
If you're doing a tech startup, the co-founder should have a technical background if you don't have it. That will save you a lot of money when you're bootstrapping as well as making sure you deploy a great product as soon as possible.
Once you want to work with someone, my best advice is to hire them to do a project or a task inside the startup. More than words, you need to see them in action and that way you can evaluate your chemistry as a team.
When you already worked the details and decided to create a startup together, you need to be smart about the plans. You need to sign a partnership agreement and put the equity in a vesting option.
In Silicon Valley, the standard vesting period is 4 years with a one year cliff. After one year, the founders fully own of the 25% of their shares. That means if they leave the company before the first year, they will not have equity in the company.
Having all that in mind, I think it will help you select the best co-founder possible and also be a better bet to investors if you ever decide to raise money. I hope it helps!