I'll take a guess and assume your body type is ectomorph, In other words; your a hard gainer.
The more good calories and odd bad calories you take in, the better. Also, protein is the number one muscle builder so look for stuff high in protein like pure chicken and steak.
However, this meens that cardio and running should not be in your regiment and those are things that are involved with muay thai.
I have the exact same body type. I weigh in generally around 145 pounds and that's what I'd fight at because I usually don't change in weight.
I trained muay thai for about 7 months and then I made the switch to a more local gym and now train as an amateur boxer.
If your goal is to just bulk up a bit, then looking up your body type is a great thing to do. The harder you work out, and get out of your comfort zone, the more your muscles will feel like their in danger/harm and want to repair themselves bigger and stronger. Resting days are big when gaining muscle mass as well. If you don't give your body a chance to rest how can it repair itself and becmme stonger.
Start eating a lot more, and start training muay thai and it will all come together bro.
" That little f**ker hit me with a Hadukan or something " - Nick Diaz on Gomi.