I mostly dive solo, even when I go diving with my usual dive-buddy.
However, if I wanted to pair....
- You could go on a dive-boat and be insta-buddied. Though that costs $$.
- There should be local dive-site and dive-clubs. Show up at any of the club-meets, or active dive-sites and people will often be happy to pair with you.
- Dive shops are sometimes start a dive-club. Because they like diving, but also because it helps them generate revenue and loyalty.
- Look for local groups based on your location, on sites like facebook, meetup.com, or other social-media.
For solo, I'd suggest taking a self-reliant course. It's not hard, but "you don't know what you don't know." Hopefully they'll cover almost everything I'd tell you myself. A rescue course is not a bad idea either, if you have the budget for it.
You should make safety something you think about on a regular basis. How you'd handle various situations without assistance, where one, 2, or even 3 things went wrong at the same time.
Redundancy is usually the solution to most scuba-problems, and often comes in the form of equipment and/or skill. I won't cover everything here, because I don't want to write a book at the moment, but
here's a good thread for that.
And for pony-sizes, I like 19cu personally, but choose what works best for you and your situation, and I have a
post on pony-sizes here.