If you need to plan a journey you will find the route and timetable information you need on the traveline website. When buying a ticket you may also want to ask for the PLUSBUS, which is a discount price local ticket for unlimited bus travel with your train ticket.
Other sources of information:
Essex has a good rail network, with most services operated by either Greater Anglia or c2c. Some areas of west Essex also have London underground stations. There are frequent services into London from all over Essex as well as to other counties including Suffolk, Norfolk and Hertfordshire.