First, go work to a quick food outlet for at least a year to see how the mechanics of it works.
Research what the most popular outlets serve and then get training to produce this product(s) EFFICEINTLY.
Research the location for your business. Pointless to say Bangkok. That is one huge area and not all the areas are suitable as quick food outlets. The adage of the three most important aspects of business are LOCATION, LOCATION, LOCATION stilll by and large apply.
You need to do a business plan. Rental Costs, electricity costs, water costs, salaries, cost of products. Deciding on your estimated gross profit and then deduce what your net profit is going to be (if you don’t know the differences of these, or how to calculat them, you need to take a business course).
The big NO, NO is to think you are special and know it all. Like any job you want to succeed at, you need to know it inside out, otherwise I predict you will fail and loose a shed full of money in to the bargain.
Rather than me saying good luck, what I will say is study, listen and learn before committing your (or Parent’s?) hard earned money.