Well National Rail gives the following route suggestion:
Ockendon dep 07:35
West Ham arr 07:56
West Ham dep 07:56
London Waterloo arr 08:25
London Waterloo dep 08:45
Staines arr 09:16
Which would get you to the park for just before 10am (opening time) once you include shuttle bus time and stuff at the other end.
However there may be some other routes and stuff, use www.nationalrail.co.uk
journey planner for them. :)
As for hotels, if you contact thorpe park they may be able to provide you with information on where to stay in the area.