That is easy. All nouns in German have to be written with a capital letter. The difficult part now for learners is to recognize what word is a noun. A noun usually is a defined part-of-speech, which can be used with articles der, die , das and ein/eine. Sometimes, however, and that makes it quite difficult, parts-of-speech can change. Verbs can be "substantiviert", meaning, verbs can be changed into a noun: Wir lernen Deutsch. Das Lernen macht Spaß. In the latter, the verb "lernen" is used as a noun. I used the article "das" and now, since it is a noun, have to write lernen with a capital letter. Ich gehe weg. = weggehen is a verb. Ich gehe auf dem Weg. = Weg is a noun. weg and Weg are two different words, belonging to two different parts-of-speech, However, the system is not so difficult and I always wonder, why even native speakers have a lot of troubles with capitalizing words. Ask yourself: Is it a noun? Then you have to write it with a capital letter.