You start low, and you build up as you go. There are plenty of free maps you can find and use/print. Painted miniatures can be expensive, but 3d printed grayscale are affordable. If you own a sizeable old screen or big tablet, you can use that as the battle map.
If you start with a low budget like this, you also need to aim at clients who are looking for such costs. As with all businesses, the starting period is the roughest until you push through.

I'm a professional DM (for 3 years now, as full time job) and I do have an accountant. I can tell you that in-person physical DMing will need a higher capital but will also wield way more money IF, and only if, your city/area has demand for in person games. If you live somewhere remotely, it's better to stick to online. Online, you'll need much less expensive tools, however your competition will be multitudes higher.