It depends
The long version:
The tools you choose will depend on the nature of the software you are testing and your clients' needs.
Some of the factors you should take into consideration are:
Budget - If your budget is constrained, you will probably want to work primarily with free tooling. This is possible, but will typically be more time-consuming than working with paid tools (although that is not necessarily the case).
Platform - The application platform will impose its own requirements. Tools that are Windows-centric often do not work well on Mac or Linux platforms, and vice versa. There is also a large divide between desktop platforms, web platforms, and mobile platforms.
Client Requirements - Your clients will have requirements. These can impose limitations on your tool choices: if your clients require PDF reports of test outcomes, you will need to find a way to provide that.
Reporting Requirements - You will need to keep track of what you have tested for which client, and your time (and money) investment in their work. You may find an all-in-one tool that will allow you to manage your testing and reporting in an integrated fashion, or you may need to build something. Either way, this will impact your tooling choice, since you will definitely want to make this process as automated as possible.
Your Team Skills - If your test team (which will likely be you and your friend to start with, but will grow if all goes well) have little experience with scripted/coded test automation your tool choices will be limited. Similarly, if your team is largely accustomed to working with Java, they may find difficulty if the tooling does not support Java or JavaScript (JavaScript may still cause difficulties because it is so similar to Java in appearance but so much more flexible).
Summary - I'd recommend you look at all these factors and aim for the most flexible set of tooling you can use as a starting point. If all goes well for your new business you can always add other tools as needed.
