Krabi is a city in southern Thailand.
The main road is quite far from the centre. You can take a bus to Ao Luek which is about 20 km from Krabi on the national road and makes an excellent hitching spot.
Take a bus to Nuea Klong. It will drop you in a village quite far away from the city on the main road. This makes a good spot to find a car to Trang.
If your driver just passes Krabi, he will drop you on the main road outside of town. But there are a lot of buses that will take you straight to the centre for a few Baht.