Tongatapu is very safe but the usual travel precautions apply. Don't flash expensive cameras and jewelry and don't leave passports, money, clothes, etc. lying around in hotel rooms. If you're staying with locals there is a good chance you'll be invited to stay at someone's home on Tongatapu take your valuables with you during the day and secure them at night
Tongan drivers are sometimes erratic so watch out. Asking for a helmet when you hire a bicycle is advisable. Dogs can be a nuisance especially in some areas outside of the capital.
Water that crashes over the reefs into the lagoons is sucked back out again through gaps in the reefs. Be careful when snorkelling that you don't get caught in one of these channels of water heading for open sea or you could be seriously damaged by the coral.
contact
There are several internet cafes in town. The Friend's Cafe is most expensive at about T$8 an hour and some places charge as low as T$2 an hour. Avoid inserting camera cards into the slots due to the risk of viruses. Use Skype to call overseas because it's about T$1 per minute if you buy a phone card.
talk
Tongan is the official language but English is very widely spoken