Start anywhere that interests you. If you have difficult memories, give yourself permission to take it slowly. Writing happy memories first may give you the confidence to tackle the painful ones. Of course, effective writing is important. For one thing, we derive more pleasure from doing things well than from being held back by lack of skill.
Writing is communication, so clarity counts.
Many new writers are intimidated at first by their lack of skill. Truly, practice makes perfect. Stifle that censor who sits on your shoulder judging every move you make. Re-writing is a key step later on. That's when you fix the grammar.