How to start going to the gym at 35 years old?

Start with light weights and basic exercises to learn proper technique - that's the most important thing at any age, that's what I'd do in your place. It's best to hire a trainer for at least a couple of sessions so they can show you how to work with each machine, otherwise you can mess up your back later.

I'll add that at 35 your body recovers slower, so don't rush to gain weight week after week. People above already correctly mentioned technique and a trainer - that'll really save you time and protect you from injuries. I'd also recommend finding a stable training schedule and sticking with it for at least three months before changing your program - your body adapts slower, but the adaptation lasts longer.

At 35 it's totally normal to start, I myself decided at 33 to get serious about sports when I realized that farming, while it does give you a workout, isn't really systematic. The main thing is not to rush and listen to your body. A consultation with a doctor is definitely needed if you haven't been doing anything for a while, someone mentioned that correctly above. After that, I'd recommend starting with light weights so that in the first two or three weeks you just get used to the movements and feel your muscles. Basic exercises like squats, bench press, and rows - that's what you need. A trainer for the first sessions isn't a bad idea, especially if you've never worked out properly before. I had one session and immediately understood where I was making technique mistakes.

I recommend not getting obsessed with heavy weights in the first month. Trust me, after a couple months of regular training you'll feel real results and payoff. For me the gym became a great way to get away from the farm routine and burn off energy. Add good sleep and normal nutrition to that - and you're done. You don't need to try doing everything like a pro right away, the main thing is to start and not give up after the first week when you get bored.