I have been battling using gen 7 OU for a long time as it was the first generation I have really gotten into competitive battling but when get 8 rolled around then most of my teams were not viable or some of the Pokémon weren't in it, so I have been stuck trying to create teams as usually I revolve them around megas which aren't in the game anymore. Is there any guide to gen 8 battling that I can use? If you want a team to do an example transform on then I will post one later.
You need to first learn the new mechanics, and these are the best mechanic-learning resources I found so far. This list is NOT COMPLETE, so please tell me if you're not okay with that.
changed moves (lists changes to power, PP, and "vice grip" becoming "vise grip", but you need to find other webpages for other kinds of changes)
changed Pokemon movepools (also includes a few notable mechanic changes to moves and abilities)
miscellaneous mechanics (this thread will almost certainly continue updating after I post this answer)
After memorizing a few of the mechanics, you can click the links on this page and this page (the first post in each of the pinned threads are particularly useful) to learn how to use the new mechanics in OU.