Hello and welcome!
tl;dr is create an account at
http://www.gempukku.com/gemp-lotr/ and spectate or play a few games online. There's usually someone around who will be happy to help discuss deckbuilding, mechanics, and run through a few practice matches. For a more in depth answer...
Gemp's automatic rule enforcement means you'll always* see how things are supposed to play. Fellowship Block is the best place to start, which is cards from sets 1-3. I never collected so I'll have to let someone else chime in on which packs to go for, but other than a few dozen rare, nostalgic, powerful cards you should be able to get whatever you want without stealing from the kid's allowance (though you may elect to anyway!)
As far as combining cards, you can pretty much do whatever you'd like. The official game plays in different formats. Fellowship Block is the first 3 sets, Towers Block is sets 4-6, while Towers Standard was the first 6 sets with the sites from only sets 4-6. Blocks tend to be in 3 set increments, with the Standard format of that time encapsulating cards from every block and only using sites from the most recent one. The beauty of having your own cards is the ability to decide what's fun and what's not. When I played person-to-person, my deck had sites from any block. While "illegal" by Decipher's standards, nobody in my group cared and it made deckbuilding more interesting - plus you didn't have to worry about violating rules you chose to ignore!
Rules also changed as time went on, but in my opinion it wasn't too significant and any old set of rules will be more than sufficient for a new player. Since you asked, though, a bit after the release of set 4, a handful of cards from the first 3 sets were banned from standard play. This started a tradition of banning or restricting to 1 copy cards which too heavily affected the game (complete list here if you're interested:
http://lotrtcgwiki.com/forums/index.php/topic,120.0.html). Towers Block introduced new rules for when Allies were at home as well as site controlling. King Block brought threats, enduring, non-Frodo ring-bearers, all sorts of things. Movie Block (sets 1-10, despite being called a "block") banned cancelling a skirmish involving the ring-bearer. Some players apply these rules retroactively (so
Frodo's Cloak can never cancel a skirmish), while some play with the rules of that time (so some later banned cards are still legal). Some players use a combination - it's whatever you like, really.
Minions and all cards borne by them are discarded when the Fellowship ends their turn, i.e. decide not to move anymore. I haven't heard of playing LotR on your own except by playing two hands at once, and I wouldn't recommend it for a new player because there's no telling if you're doing what you're supposed to. You can create 2 accounts on Gemp and pit them against one another, and that will prevent you from doing anything illegal but it won't help you learn how to play any better and avoid silly mistakes.
Again, I recommend playing against someone willing to teach and after 3-4 games you'll have a solid understanding of the gameplay. Specific scenarios will come up and you won't know what to do, and that's what we're here for! My final unsolicited advice is to do your best to let your son win without obviously throwing the game. Once you get a good grip on winning and losing, make your deck weaker than his and be sure to include his favorite companions. There are many players around here with more cards than anyone wants to bother counting, and I know you could get a steal on a ton of extras - someone recently gave their modest collection away, shipping and all, so that the cards could continue to be enjoyed. Check out the marketplace if you're interested:
http://lotrtcgwiki.com/forums/index.php/board,7.0.htmlOnce again, welcome to the community! Like you, I
discovered this game years after it had ended and have enjoyed it more than almost
any other game. From shooters to racing to strategy and board/party games of any variety, I haven't found a game with so much depth and freedom available to the player.
*there are a handful of highly disputed fringe cases and coding errors, but for a new player these aren't even worth mentioning