If you want to party, head south. Otherwise, search for info on Thailand. There are lots of things you can do and this will give you ideas. Thailand is a beautiful country. I personally loved the North. If you go up there, you can go into Laos and return to Thailand when it's time to renew your visa.
If you do what I suggested about researching the rough cost of things to get a rough monthly cost then you will have an idea for your budget. The how much money do I need is like a piece of string. It might be better to tackle it from this angle: What is your maximum budget for the trip? That's the way I did it and I made sure I stuck to it. We all have budgets, which can vary from very little to a lot of money. It just depends on what yours is. It will be different to other peoples. I know it's hard. I had to think about it for my RTW trip for a year. Thinking about that was really difficult before I left. So, I had a maximum budget for the year and when I ran out, I had to go home regardless. Budgeting is a great skill to have and will be one of the many you hone while you are away!