Planning a 3 month trip with friend to Thailand/S.E Asia for next fall. Have a few questions.

So my friend and I are still in the early stages of planning but are trying to nail down some big basics first. Any advice or pro tips would be greatly appreciated!

Some of our initial questions:

• What are some of the best things to see
• What are come places to avoid
• Any off the beaten path/non touristy gems you know about?
• how important will it be to study up on a few languages or will simple basics suffice?
• If any one has done a similar trip, did you use an itinerary and how did that pan out
• any cultural habits or social courtesys we might be ignorant to?
• should I be preparing my stomach in advance for any certain foods? Aka spice.

And any other tid bits you might have. Thanks so much!!