Dining customs are where visitors most often stumble, because the rules are invisible until you break one. The safest universal habits: wait to be seated or told where to sit, wait for the host or eldest person to start eating, keep your phone off the table, and compliment the food.
Tipping is genuinely country-specific and worth researching before you land. In North America it is close to obligatory, across much of Europe it is a modest round-up, and in parts of East Asia it can actually cause confusion or mild offence. When invited to a home, bringing a small gift such as sweets, fruit, or flowers is appreciated in most cultures, though the specifics of what to bring vary.
As a guest, expect generous hosts in many cultures to press second and third helpings on you; a polite initial refusal followed by acceptance is often part of the ritual. Finishing versus leaving a little food on your plate also carries different meanings by country, so observe your hosts and follow their pace.