Get in the saddle and ride. Preferably outdoors. It’s more interesting. If you can do bicycle only paths, that’s easier and more fun. Do not be afraid of hills. Just use your gears. You may go very slowly up a hill, but you will go quickly down the other side. Just keep on peddling.
Start small. Maybe five miles. Do that for a week. Every day. Move up to to ten miles. Keep on increasing. If you can’t do every day, do what you can. Once or twice a week. If five miles is too easy, do more.
Don’t be afraid of sweat. If you are going to be out in the sun in the middle of the day, use sun screen. Make sure your bike is properly maintained. Learn to do your own bike maintenance, too. Take a class at a local bike shop if you don’t already know this.
I think clothing doesn’t matter that much. I’ve been wearing shorts and t-shirts and sneakers to bike in for 50 years. No need to spend money on anything specialized.
I just warm up by riding. I start slow, and then as I warm up, I start putting more effort into it. It’s a matter of staying in tune with my body and not pushing too fast too soon. I also do a lot ankle stretching afterwards. Biking tends to tighten my achiles heel tendons.