Level up a seasonal character from a class which you always wanted to try. Get a hold of the game again.
Finish the story mode and get to max level. If a game is too easy you can adjust the difficulty in the settings when you logout. If it's too hard, simply put it on easier again (this can also be done when hitting Esc while playing, top right corner).
Meanwhile while you're completing the story mode, get a hold of the new Mystic introduced in the expansion. This is a new ''profession'' next to the Blacksmith and Jeweler.
The Blacksmith can still be useful by using Kanai's Cube (will explain later), to upgrade a rare item to a random legendary you'd like. For each class it's different and it all depends on what builds to play but overall this is the only part you use the Blacksmith. Also, this is the reason why you don't sell items (besides it being faster), but just destroy all items at the Blacksmith for reagents.
Nothing changed much with the Jeweler, it just has the same gems except for a new All Resistance / Cooldown Reduction gem. But you'll find out yourself.
The Mystic is new, you can change your look here (cosmetic) and change stats here. At later level I'd suggest to try and find a good build which you like and read up on the best stats for that specific build. That way you don't ruin good rolls on legendaries and hate yourself later. If you just want to have a fun time though, just mess around with it.
I'm not sure but I think once you reach max level you get a quest to get Kanai's Cube. This cube is really powerful since you get 3 traits from 3 different legendaries, like for example the legendary trait from Flying Dragon is ''Double your attack speed''. When you loot an item like this with a trait that you like or is good for your current or future builds, don't destroy it but save it to later put it in Kanai's Cube to get the trait without having to wear the item. You can use a trait from a weapon, amulet or ring and all other gear pieces. So 3 traits in total without having to wear the legendary. You can notice a trait on a legendary when it is displayed in orange letters on the item. You can also do more stuff with Kanai's Cube but you'll have to look that up yourself and most will explain itself.
At max level you'll also find yourself getting Paragon levels, there is no max level on this. The way it works is that you need more experience every higher level. If you want to spend your points, press ''P'' for the menu to pop up or simply press the blue button just above your action bars which always appears when you get Paragon points to spend. It doesn't really matter if you screw up by spending the wrong points, since you can just press ''Reset'' to reset the points. Also, if you have more points to spend, shift click on the points to spend 10 points per click instead of clicking 100 times you can just click 10 times.
When you're done with the story, you can start doing bounties. Press ''M'' and use right mouse button to zoom out on the map to get an overall view of the bounties. On the map there is displayed one act with ''Bonus''. This is the one you'll want to go since when you complete all 5 bounties you don't get 1 cache but 2, which is all you want. The caches contain a lot of loot, blood shards (which you can also trade in at a vendor Kadala for loot. Always nice if you're looking for a specific item like if your gloves suck, just keep on buying gloves and you'll find a better one in no time.), and specific items you need to use in Kanai's Cube.
Nephalem Rifts. These can be done by clicking the pillar and going for the option Nephalem Rifts. This is mostly run for getting Greater Rift keystones. Read on.
Greater Rifts. The one thing the ladder is measured upon, either solo it or go multiplayer and try to get the highest rift possible. The higher you go, the more loot and experience you'll obviously get, but also the chance on upgrading a legendary gem increases. What are legendary gems? These are new gems which you can only put in your 2 rings and amulet, so you can wear up to a maximum of 3. Which you always want to do since they are really good. So for example, if you don't have a socket in your amazing ring you just looted, you go to the Mystic and reroll the worst stat on that ring for a socket in your new gear piece. This is luck based so you might need a few tries before this works. When you got the socket, simply put your legendary gem in and do this until you got both rings and amulet socketed with a legendary gem. Then you can upgrade them at the end of a Greater Rift (if you make it before the 15 min timer runs out!).
I just copy pasted a post I placed a while ago, hope it helped. Have fun!