The best resource for learning comprehensive strategy is Brian Rose’s free Othello ebook- http://www.ffothello.org/livres/othello-book-Brian-Rose.pdf
The US Othello Association’s official website is usothello.org (surprise! you’re here already!), it has some resources as well, including a blog (self-referentialism for the win!). Tournaments are usually listed on this domain.
There is also a Facebook group page for the USA Othello association located here: https://www.facebook.com/groups/178039435681/, we announce upcoming tournaments on that page as well. The Facebook page also has Othello puzzle posts which you may enjoy. Sometimes there are Othello lessons being organized there, too.
There is also a mailing list firstname.lastname@example.org. It is also a good way to potentially make new American or international Othello friends/connections.
Currently, the tournaments for 2017 have not been announced yet. However, there are usually three qualifying tournaments for the world championship — one in the west coast, one in the east coast, and one in the midwest. This year these tournaments will probably be in Los Angeles, Chicago, and New York City.
If you Google “Reversi Wars” that is the most active game server for Othello. It doesn’t have any chat, but it does have guaranteed games against someone near your level of skill. It works best on Android or iOS devices, but also works on PC.
And if you want chat, the website PlayOK.com > Reversi is the most active as far as good players are concerned.
Ben Seeley plays on both sites as “foompykatt”, he is a former American world champion. There are other strong American players active online as well.
If you want a strong program to play against and study games- Google for “wzebra” for Windows or “droidzebra” for Android. If you have a Mac, Google for Stephane Nicolet’s Cassio program.
If you want to study tournament games, the international databases are located here- http://www.ffothello.org/informatique/la-base-wthor/ (just use Google Translate to translate the page from French).
The ratings list of top active tournament players is located here: http://ratings.worldothello.org/ratings_main.php
The wikipedia page is located here: https://en.wikipedia.org/wiki/Reversi
Pisuke is a nice endgame trainer app on Google Play. Pisuke costs like $3, everything else I have mentioned is free.
The website woc.worldothello.org has games from the history of the World Othello Championships.
The Japanese website othello.org has a daily Othello endgame puzzle, too, that is quite popular. If you use Google Translate it is easy to find the puzzle on the front page.
That should get you started, please ask if you have any further questions :).