If you are arriving into a new season a few days late, then you might be able to catch up a lot easier by getting a powerlevel / boost in order to reach level 70 very fast. A geared level 70 player can typically get a full party to level 70 in under an hour, and there are actually a lot of players who are willing to boost others!
If you are looking for a boost, then open the Communities page and look for a powerlevel/boost Community. There are many of them, and often there are hundreds of boosts online. Just ask nicely, and someone will usually boost you very soon.
The booster should simply open a new game on Torment 6. This is the highest Torment level a new player can use. He should then rush trough normal rifts, and 4 or 5 tend to be enough to get the new player up to level 70!
The person who receives the boost should simply be waiting at the entrance, and you will get all the XP. If the booster changes floors, use his portal to get to his current floor, or else you will not get any experience.
Remember to give a big thanks to the booster once you hit level 70!