Get online and start the application. Upload the documents yourself.
Find out the nearest place for your wife to have a TB test and English test.
That's the start. You'll need to book an appointment at VFS too.
If memory serves me well you can upload documents even after you have completed and paid for the application. Therefore, if Bangkok is where the TB and English test need to be taken, you could do these at the same time, make it a 3 day trip, as you are in Bangkok to visit VFS.
Note, if you use an agent you still have to go through the same process. All the agent will do is check you have the correct documents. You can do that yourself by downloading the UKVI checklist.