[New Friend Greeting] Long-sitting Grad Student & Heavy Eater Decides to Start a Healthy Lifestyle, Seeking Advice!

Hello everyone! I'm so happy to join this positive little space and connect with you all. Let me briefly introduce my daily life: I'm currently a graduate student who spends all day in front of the computer running data and writing my thesis. Because I often have to rush deadlines, my meals are almost entirely reliant on delivery. Fast food chains, fried items, potstickers, and boxes of instant noodles from online orders are basically my standard living. I've recently realized my stamina is nowhere near what it used to be; I get tired after just a little movement. So, I've decided to seriously adjust my condition. However, as someone who eats out heavily, I'm a bit lost on where to start with a "healthy lifestyle." I'd like to ask the seniors here: for someone like me who can hardly cook at home and often has to sit for long periods, what would you recommend as a first step to adjust for noticeable results and easy adherence? Should I start by slightly tweaking my delivery food choices? Or begin with simple home exercises every day? I'm looking forward to everyone's sharing and advice. I hope to regain my health along with all of you!