I'm not going to give any advice on the 'actuals' of what to do (I'll be honest, the last place anyone should be taking fitness advice from is an airsoft forum) but I will pop some things that are worth thinking about below:
1) Though generally they go hand in hand, fixing your nutrition should be a higher priority than exercise. Changes to your nutrition should be sustainable, fad diets are all well and good for getting quicker results but are rarely ones most folks would be able to maintain for an extended period of time.
2) Though I'd tend to advocate talking to professionals, an easy way to start amending your nutrition is plugging your details into a TDEE calculator. It won't be 100% accurate but it gives you an ok starting point and calculating where you want to be for a calorie deficit. Cutting out fizzy drinks and replacing them with water is all well and good, if you're still taking in 4000 calories a day in other junk with shit macros then you're likely still in caloric surplus... and such, putting weight on.
3) Your motivation will be the first thing to cave, your willpower is going to be tested more than anything else in your body will be. Be mindful of this.
4) Starting point of folks health and fitness will vary, be careful following whatever the hot thing is on the internet. As much as folks will try to sell you on 'this works for literally anyone', that's not necessarily the case and linking to the previous point... all it'll take is falling at some early hurdles to wipe out your motivation.
5) Know what your goals are, if you go into it with 'I just want to be a bit healthier lol' it'll be really hard to maintain motivation as time moves on because you'll quickly realise you're not working toward anything and that initial drive you had at the start will fade.
6) You get out what you put in. Some would rather make non-intrusive small changes, others are looking for a lifestyle overhaul. Either way, the results are going to line up with what you change and how well you stick to it.
7) The biggest one, the one we've all done at some point in time... and will likely all initially claim we haven't. Stop making excuses for not doing things.