First of all as written in the Parent Page (Participate) the relevant tags you should know as a Mtbiker in OSM are written here: http://wiki.openstreetmap.org/wiki/Mountainbike
If you're a beginner unsure about how to enter that data into OSM here is a quick help (for further help check OSM Potlatch / JOSM wiki.), Potlatch is the editor that you will see when you click on edit on openstreetmap.org, Josm offers much more functionality but is a bit more complicated and is a seperate program (requires Java Runtime):
Josm now includes mtb:scale in the presets for pathes and tracks. For incline and mtb:scale:uphill; mtb:description; mtb:scale:imba or others, you can add the keys and values by clicking on "add".
For keys that are not available on a preset you can either add a mtb preset from here: Josm Presets or directly entering the information by clicking on "ADD".
ID - editor
ID is the new default editor on openstreetmap.org. After a few years development it now actually got quite useful (though not as powerful as JOSM).
Heres a walkthrough with pictures:
The default layout is again only for beginners and very confusing for anyone wanting to edit MTB stuff. So we click on "All Tags"
and find a nice list with all the tags on the object. to add mtb:scale - just click on the "+" button.
If you click on the "i" button - for many keys you get a direct link to the wiki to find out their meaning. This however is still a bit patchy and doesn't show all keys that are defined for OSM in the wiki.
Very Important - if you add incline key in order to specify the direction of the way. Look at the tiny arrows on the way/line to know the direction.
With '''Potlatch''' after selecting a way click at the bottom onto "Advanced"
And then by clicking onto "Add" you can add any key you like plus the value.