@azdr0710 You'll have a great time I'm sure.
Air travel in SE Asia is easy and cheap and very frequent. I would consider air service in Asia to be heads above our domestic service. The airports and airlines are some of my favorite in the world and put the US ones to shame. Air Asia (a Malaysian company) offers exceptionally cheap fares between most cities, but even the more expensive airlines are relatively cheap when traveling in Asia. When flying into Bangkok double check which Airport you are arriving and departing from (there are 2).
Buses can be hit or miss depending on country. I would go for the nicest option offered (normally called VIP) as you don't really want to end up on the locals bus, but even then it can be quite the adventure, bring something warm to wear on the bus and snacks.
I've not taken a long train journey but have been told they are some of the most beautiful in the world. Still flying is going to be the easiest and most convenient in most cases, and not that much more expensive. Book in advance for the best fares.
Download Grab Car. It's basically Uber in Asia (although they have both Uber and Lyft) but the rates are cheaper for taxis.
Thailand is so easy for tourists, too easy if you ask me. It's definitely worth seeing the major tourists spots, there are really no "bad areas" in the center that I can think of. I have not stayed at any of the hotels mentioned as Airbnbs are very cheap and abundant in Bangkok, normally in nice residential areas. Make sure you are within walking distance of the Metro or BTS Skytrain though. Taxis in Bangkok vary greatly and aren't worth the hassle most of the time. You can take the train to and from BKK and go to the popular places in town on it without dealing with the oppressive Bangkok traffic. Don't bother taking a Tuk Tuk, if they are not a scammer trying to get you to their friends suit/gem/whatever shop they will gouge you on the fee. Visit Pat Pong if you're interested in the infamous Bangkok nightlife, I found it kind of tame actually, a little too commercial. But if you want to sample the wares there are a couple places I can suggest depending on your type. Eat the street food, it is great. Khao San Road was stupid to me, but worth visiting just to see all the drunken revelers. Tours and travel can easily be arranged to other parts of the country like Chiang Mai, Phuket, etc. but check to see what the weather will be like when you are traveling to the beaches. Local travel agent booths are reliable, be more weary of online services but if they have good reviews go for it.
Other Places - All depends on your interest I wouldn't say any country is "not worth it. Cambodia, Laos, Singapore, Malaysia, Borneo, Vietnam, Philippines, all have tons to offer. Myanmar, is also a good place to go if you want to get off the beaten trail and see something a little different.
Is it possible to pick up some reward points/airmiles before your trip? Some of the Airline CC or Chase cards offer really good rewards bonuses that could cover a significant part of the over the water flights. The airline cards also tend to come with travel perks like lounge access, or priority checkin