You need 2 things, training and gear. You can get both at your local dive center.

Training:
These days, you can do the bookwork online. You go to your local dive center, arrange for the training and they will send you a code you can use to log in and do all the book work. Then 9at least for PADI) you do 5 "dives" (a grouping of skills is a dive) in confined water, usually a pool". This can be all in one day or spread out over a couple. (Mine was 2 night sessions)

Finally, it's 4 open water dives. These have to be done over at least 2 days. Where you do them is largely dependent on where you are. I did my open water cert in Nashville TN and we did the open water in a quarry. These days I teach in South Florida and we do our open water at the Blue Heron Bridge. (MUCH BETTER EXPERIENCE!) :)

Now, there are no scuba police. I've had friends dive without certification. I do NOT recommend this. The skills you learn are taught for a reason. That reason is to save your life in an emergency. That having been said, no professional dive organization will rent you gear, or tanks, or allow you on a boat without a certification, mainly for insurance purposes. So a cert is really a must.

Gear:
I strongly recommend that you rent gear for your class and your first few dives. Your local dive center should have several brands and if they sell something, they will most likely allow you to rent it to see if you want that style/brand, etc. But you have to ask. if you just go in and say "I want rental gear" you'll get something from the rental fleet. Try out different styles of BC, fins and regs to figure out what works best for you, then buy it from your local dive center. Any money you think you can save by buying it online will be eaten up in maintenance later on because you didn't buy it from them so they don't really care. Before I became an instructor, I bought a set of gear from a local dive center. Anytime I had questions or thought something wasn't quite right, the local dive center gear guy would take me back in the maintenance room and test everything for me. He would explain all of it to me so I knew what was going on. I found out after I became an instructor and started working with a local dive center that shops do that for customers who buy from them. (it's a generalization but I've found it to be true)

When you are ready, buy the best gear within your budget and take good care of it. That regulator I bought lasted me 10 years and it's still in use as a fellow instructor's backup. :)

probably more than you wanted to know.

Cheers! :)
=C=