By far most nouns take -en in the plural. The formula is very simple:
noun + en
However, when you add -en to a noun, you have to bear in mind three spelling rules:
1. Maintain the length of the vowel
Short vowels should stay short, long vowels should stay long, regardless of whether the noun is written in its singular or plural form.
See also keeping vowels short/long.
2. No open double vowel
If the last syllable of the singular form contains a double vowel, be sure to avoid an 'open double vowel' in the plural form.
3. Replace hard f/s by soft v/z
If a singular noun ends in hard (unvoiced) f or s, we generally write soft (voiced) v and z in the plural. See also hard and soft consonants.