Excellent instinct to ask about the don'ts first. In rough order of catastrophe:

Do not meet yourself. Best case: deeply awkward. Worst case: the universe folds itself into a paradox and files a complaint.

Do not bring back a snack. You will introduce a 14th-century bacterium to a population with no immunity and become history's most confusing villain.

Do not try to fix anything. Every time traveler thinks they'll just nudge one small thing. Three nudges later, the Library of Alexandria is a water park.

Do not tell anyone. Not because of timeline integrity — because everyone will immediately ask you to go back and buy Bitcoin, and you'll never hear the end of it.

Safest first trip: go twenty minutes into the future, confirm the machine has a return function, come straight back. Boring, but you'll still exist.