Really trying to think of a usecase here.
Big companies already have their GIS systems in place, often proprietary, with tailor-made extensions, tying in to all their other systems.
They pay a lot to create this data and to keep it accurate and up to date.

This dataset is very closely connected to their core business. As a rule, you do not outsource your core business.

So OSM sounds lovely, but it isn't.
- you can't rely on other people to map area's or changes because you don't know when they'll do it.
- you have nothing to say about the data quality. People will not follow your standards.
- The system doesn't connect to all your other systems
- Most importantly: You already have a system with high quality data in place. Why would you change it?

I can think of two applications though:
1. They can donate data. It'll need to overcome some internal resistance, but especially public companies may decide that their data is publicly owned anyway.
They can help to improve the quality of OSM data, which in turn may improve the quality of commercial data (by cross-checking) and lower the commercial prices (by competition).
2. In 3rd world or otherwise unmapped countries where there is no data available yet. OSM may be the system with the most data available.

Nice way to start:
Use your GPS-tracer. Walk a short path so you know exactly where you walked.
Upload the GPS-trace to the computer.
Open the GPS-trace with JOSM. Download the map-features that exist already.

This was preperation. Now you have to find out how JOSM works. It's new at first but you'll get it.
Set yourself a simple task: To draw and upload the path you walked.
Watch videos, read manuals, and try until you get it right.

When you feel you can confidentily make small changes, start to work bigger. Eventually you can draw anything you like. Everything in JOSM, from the smallest footpath to the largest hospital, is just ... nodes.