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".
With '''Potlatch''' after selecting a way click on the "+" icon:
And then replace ''key'' with the respective key, and enter the value into the gray ''type value here''