How do I know which ending to use for the plural of German nouns?

You can't always know which plural ending to use just by learning a noun and its gender. That means that you also have to learn the plural ending with the noun when you learn it as a new item of vocabulary. However, certain endings and genders are associated with certain plural endings and you will hopefully get a feeling for these connections as you learn the language. For example, the endings -ant, -tät and -keit/heit are always pluralised with -en, but no plural ending is added to -chen. Most of the time, the masculine plural ending is -e or nothing (sometimes with an Umlaut added to the vowel), the feminine plural ending is -(e)n and the neuter plural ending is -er, -en or nothing, but this is not a hard and fast rule. The plural ending (and gender) of a compound is always the same as that of the last part, e.g. Haus - Häuser, Rathaus, Rathäuser. Remember than an -n is added to the plural ending in the dative plural if the plural ending does not already have one (except for plurals in -s - no extra -n needed here). This -n may or may not be part of the plural ending, e.g. der Stuhl, die Stühle, zwischen zwei Stühlen, but die Katze, die Katzen, mit den Katzen.