The low level rifts don’t really give much gear and you need set piece gear to do them, yet the higher difficulty levels are pretty much impossible when you dont have gear!
Legacy of Dreams gem. This tends to be the second gem you’ll find from doing Greater Rifts. All you need to accomplish is wearing all orange gear (the legendaries), and try to have no set bonuses. You can have set pieces on, but make sure to have no more than 1 piece of any set. At that point, you should be strong enough to play T7 difficulty with relative ease, and level up the LoD gem. Just kill stuff until you have a full class set.
To easily get your legendaries, just run bounties. The difficulty doesn’t matter, just that you ran them. After 3 full sets of bounties, you should have every craftable item. There is at least one legendary (orange) per gear slot to help get you geared up to use the LoD gem.
There is also judicious use of they mystic to enchant your gear, plus using the cube, Kadala, and even crafting to help you along. Also, make sure you pay attention to the legendary gear: read their descriptions carefully, and make sure you are actually utilizing them.
There is also seasons. You get a free set via Haedrig’s gifts, which will help you along immensely.