Quick answer
Safety comes before evidence collection. Reliable existing records can help explain a pattern.
Do now
If safe, record dates, specific conduct, witnesses, injuries, and reports. Keep originals, surrounding message context, and medical records. Ask an advocate how to store copies outside shared accounts and ask a lawyer about lawful recording and court submission.
Watch for
Do not provoke contact to obtain evidence, break into accounts, or assume recording laws are identical across state lines. A lack of perfect documentation is not a reason to delay asking for help.
Official sources
NJ Courts — Domestic Violence, NY Courts — Domestic Violence / Family Offense, DC Courts — Domestic Violence Division, National Domestic Violence Hotline — Internet Safety.