I know I don't have that many followers, but I got this info from someone who does.
People ask me this a lot, and I asked it a lot too, at the beginning. It's sometimes difficult, but people just makes it difficult.
There are multiple ways of doing this, and some like one way more than another, so just choose the way you like!
One way is to join things like awards, and communities, or even creating them. Be aware that this is a commitment, and it takes time. And, since I know that people don't have that kind of time, I came up with another option.
Here's the second one: Follow people. Most are nice enough to follow you back. Note: If you follow lots of people, no more than 300 at a time is probably a smart choice. It's because you can see how many people your followers follow.