Welcome to the forum! I think everybody has a different concept of what they want to bring with them on a bike ride. For health and safety reasons, water or other hydration is pretty essential, especially during the hotter months. Many riders have two water bottle cages on their bikes and have a water bottle in both. There are general guidelines on how much water you should be drinking (something like a liter per hour, or 20 oz per hour, something like that). I'm sure you can find differing suggestions online, but they'll all be more or less similar: drink, and drink probably more than you think you should be drinking (water, that is).

As far as other tools and supplies, most people will carry a spare tube or patches and a pump, so they can fix a flat while on the road. If your bike rides are relatively close to home, you may not feel that to be necessary, but even if you're just 5 miles from home and you get a puncture that you can't fix -- that's a long walk home. Certainly, as your distances grow, you'll probably want to carry some of those tools and spares. You can carry this in a saddle bag (mounted under the saddle) or in a trunk bag on a rack or in a water bottle-like container in one of your bottle cages. I carry my stuff in an SKS Cagebox, only because I have a few different bikes that I ride, and I just buy my tools and prepare my tool kit once, and then just bring it along on whichever bike I'm riding that day. If you do have or prefer to have one bike, something like a saddle bag may make more sense to you.

Depending on the length of your rides, you may want to have a rack and a trunk bag, so you can pack food and/or tools and/or clothing for longer trips. I have a rack on each of my bikes and, if I'm on a longer trip where I want to carry additional water or food, I'll put the trunk bag on and pack that trunk bag. I still have one water bottle and my tool kit in the two bottle cages on the bike, but additional water is in the trunk that I can switch out when I use up the first bottle.

As far as other things, you may find that you want to replace the saddle or grips or add a cycling computer so you can track your miles, etc. As you continue to ride, you'll discover what works for you and what's not working and needs changing.

And, of course, if you don't already have one, a good helmet.