Karatu is a dusty little town that lies on the gateway to Ngorongoro Crater. People here are accustomed to seeing tourists and many of the children will run up to a foreigner and ask for school supplies, or money for them. They are given school supplies that are at least basic, but may barter them for other goods or money. Learning the local greetings assists as far as kindly bypassing that and re: communication in general. A simple saitaa will cause recognition, and a conversational response. If you say this, it is best to know more than one word for that reason.
The cheapest and easiest way to get to Karatu is by Peugeot. You can find them at the main bus stand in Arusha. Prices are 6,000/= per person or 45,000/= per car. A car will hold 8-9 people and if you're paying per person it will not leave until full. It is normal to pay upfront so that the driver can buy gas.