Got a side arm... but what next ?

The next gun you should buy is a Rem 870 in any gauge you want, but I'd say with 20 or 12 just for ammo resupply.

Next would be a good .22 rifle of any make you like. The Henry AR-7 take down rifles I've seen don't work, from well to at all, and returning them to the factory doesn't seem to help. That still leaves a lot of bolt, lever, pump, falling block, rolling block, semi-auto and more to choose from. The Ruger 10-22 is about as common as you can find and works very well and has more accessories than you can count in a day. However there are many excellent used Rem bolt action .22s out there that are cheap, as well as Mossbergs, Hi-Standards, Marlins, etc, etc.

Join the local IPSC or IDPA club in your area and learn to shoot. Neither group is perfect and both have a lot of silly rules that make no sense, but any trigger time you get "under the gun" (shooting against others against a clock) is good trigger time. If you local club treats you poorly, there are lots of others around. One club 90 miles away has a well earned rep for treating anyone not in the "core group" as dirt, but there are four others in a 90 mile radius that I can go to, one every weekend, including one 30 miles away that treats everyone well.

 

"Join the local IPSC or IDPA club in your area and learn to shoot."
Good advice, but I'm lucky enough to have grown up shooting. My dad owns a pawn shop, and every gun that ever came out of pawn for sale, we had to test at the range... he was anal about it. Can't think of many guns I haven't got at least a little exp. with. But, you can never practice enough... especially with 'your' guns specifically.
I recently had to sell my guns due to a financial bind, and just now able to start purchasing again.

 

They weren't kidding about the 10/22 accessories. Check this out. as for a .22, my choice would be a Winchester semi-auto. (I can get the model number if needed). My Sister has one that eats any standard .22 from shorts, to .22lr mix and match, it like them all and has never jammed, even when shooting mixes loads. My Glenfield #60 on the other hand take only .22lr, but they are fairly cheap, reliable, and they made more than 12 million of them, so they are not too hard to find, and it is not hard to field strip and clean. As for shotguns, the 870 and the 500 are similar in that they can swap out barrels easily without tools. My Mossberg 500 12ga, has the smooth field barrel and the sabot barrel. More barrels are available. Raised rail and bead sights at muzzle and at mid point, make aiming quick and easy, though mine is scoped for the sabot barrel at present. Very reliable, and It has never jammed, or mis-fed.
as for size, I would suggest you stick to 12ga. the selection of ammo is best, the variety is huge, you can even fire flares through a 12ga, without damage to the 12ga (uses only the primer) even parachute flares. And 12ga. is available nearly anywhere. Good luck.