Can you please tell me, should 100mg of testosterone help me build a decent amount of muscle and burn fat?

Maybe but it depends: Are you asking about a 1 time dose? Or are you committing to taking it regularly? And if latter, will you be getting your levels & labs monitored & perhaps your dose adjusted? And will you be willing to eat right & exercise regularly? Lots of variables to this. Furthermore, what's your level now? If it's in high normal range, just start eating right & exercising. If it's low or low normal, maybe.

No amount of test: If you are not stimulating your muscles with heavy resistance training, no amount of test will help you build a "decent" amount of muscle. There may be a small amount of muscle produced from 100mg of test because of increased protein synthesis, but not much without weight training. Same goes for losing fat. Without adequate exercise, test by itself will not cause a "decent" amount of fat loss.

Testosterone: First of all, you need labs to understand where your testosterone level is. If deficient or low-normal, testosterone supplementation may help you build muscle. But the only way you will get a decent amount of muscle is if you also work out. If you do build significant muscle, it will help you burn more calories and so you may be able to lose fat. I hope this helps!