It is really quite straight forward. Lets take for example I want to add a parking lot as POINT so the parking lot symbol will show when rendered with the standard mapnik renderer aka the main map you see when first starting to us OSM maps.
- Register for a free account here you need this to do editing.
- Zoom to a small area where you want to add some data using the map interface. If you can see your entire city that is to much and is not allowed, try zooming in on a smaller area it will be faster too.
- Select the online editor by just holding your mouse over the Edit Tab and select the online editor for this example I'm using Potlatch2 (You need FLASH installed) .
- Now double click one time on the sport where you want the parking symbol to show up. Then select the type of object using the drop down list and select "Car Parking".
- Add a Name like "guest parking" if you don't know it you can ignore it.
- Add the number of spaces if you know how many there are if not just ignore it.
- Click Save in the upper right hand corner and add a simple comment like "added parking spot"
Please feel free to let me know if this works out for you.