1) Relax. Do NOT panic or freak or stress out. Stuff has changed. Hopefully it has changed for the better.
2) Things will be broken and/or screwy. Accept this. I would not recommend attempting anything major, like progression raiding, for a day or so. Maybe go do a fun run or two. That should help you get used to the new way of things while giving you a nice cushion for failure.
3) Update your addons. While you're at it, figure out what addons are musts and update them and get alternatives for them. For example, one of my musts is vuhdo. If vuhdo is borked, well, I know that my options include healbot and grid/clique. It's also a good time to see what addons you can live without, and a good time to check out the changes to the default ui.
4) Your class has changed. Find info on said changes. Here's a lovely compilation for patch 4.0.
5) Don't overwhelm yourself! If you have more than one toon that's changing dramatically, focus on one at a time. Allow yourself to process and understand the changes on one toon before moving to the next.
Again, relax. Don't stress out too much. It's a game. And to hopefully make you smile and remember why you love wow I give you random awesome machima!
Boom De Yada
Lament of the Highborne
Big Blue Dress