Don't worry about trying to memorise these if they're new to you.
You'll quickly get used to them as you read. The text at [
http://www.snerpa.is/net/isl/njala.htm ] isn't completely
assimilated to modern conventions, so I'll just mention some changes
that might be relevant. Actually some of these changes go back to
the Middle Ages, so this is more a list of differences between
modern spelling and the normalised textbook spelling of Old
Icelandic based on how the language was in the early 13th century.

1. The ending -r > -ur after consonants, e.g. maðr > maður, Hildr >
Hildur. Also where -ur is part of the root, e.g. fagr > fagur,
fagrt > fagurt. This doesn't apply where a vowel follows in the
ending (nökkrir > nokkrir), except with the suffixed definite
article: maðrinn > maðurinn.

2. Final -rr > -r, e.g. optarr > oftar.

3. The combination pt after a vowel is often now spelt ft, e.g.
optarr > oftar, aptr > aftur.

4. Final -ss, where due to assimilation of s + inflectioanl r, > s,
e.g. íss (nominative) > ís, but final -ss stays in the genitive,
íss "of ice" > íss.

5. In unstressed words and endings, final k > g, final t > ð. E.g.
ok > og, ek > eg/ég, mjök > mjög, lítit > lítið, at > að, hvat >
hvað.

6. The sequence vá > vo. E.g. várr > vor, hvárt > hvort.

7. tvau "two" > tvö. sjau "seven" > sjö.

8. In a few words, e > é. E.g. fekk > fékk, fell > féll.

9. þykkja > þykja.

10. The 1st person singular past ending a > i, so no there's no
difference between 1st and 3rd persons in the past singular,
e.g. "síðan vaknaða ek" > "síðan vaknaði eg", "ef ek væra" > "ef eg
væri".

11. The subjunctive endings im, ið, i > um, uð, u (as in the
indicative).

12. z > s (ss between vowels), e.g. unz > uns, Gizurr > Gissur.

13. The endings -sk and -zk > st.

14. oe > æ. E.g. broeðr > bræður.

15. In some words, ø > e. E.g. kømr > kemur.