I'm the opposite. I'm 6'3, and was around 150 - 160 during high school and university.
I'm now up to 195. Weight gain (and weight loss) is all about calories. Find out what your TDEE (Total Daily Energy Expenditure) https://tdeecalculator.net/
If you want to lose weight, eat 500 calories less than your TDEE. Do the opposite if you want to gain weight.
Download the My Fitness Pal app on your phone. It's easily the best tool to track your calories, as you can simply scan the barcode of any food package to input all the data. Incredibly quick and simple.

While losing weight is probably 80% diet and 20% exercise, exercise is still very important. Please don't pay for a personal trainer, it won't be worth the money.
There is so much valuable information online. It may get overwhelming, but I'd highly suggest Athlean-X and Jeff Nippard on YouTube. All based on science, no bullshit.

I'd love to answer questions if you guys have any. I love this stuff.