Your hero/heroine has now left the ordinary world, accepted - reluctantly or with enthusiasm - the call to adventure after first refusing it, has found a mentor who will help them with their journey and crossed into the other world.
Now's the time to throw your hero to the lions - or the tigers, floods, sandstorms, blizzards on the world's highest peak or a hurricane on the Atlantic. Tests can be physical - storms or tigers - or they can be something else. If you're writing a book where the journey is internal, then the tests will be so as well. But remember, the most important part about these tests is that they force the hero to face their deepest, darkest fears. Tests, physical or not, have to be crucial to the hero. They have to mean life or death to the hero. So if snakes are your hero's deepest fear, you need to make sure that they face the snake. If your hero's deepest fear is falling in love and being rejected, make sure the test reflects that.
But along with these tests, comes the fun part - at least for you as the writer. Because this is where you get to create the evilest villains and the best of friends. Secondary characters are often much more fun to write than the protagonist/hero because they can be funny or lighthearted or eccentric. If they're villains, you can push them as far as far. Think about Lex Luthor or the Joker or Ernest Blumfeld. Those villains are as evil as evil can be. And because in those cases, they're caricatures, they're funny as well.
If your book contains an internal rather than an external journey, you still have to include the enemies but in that case, the enemies could be your hero's fears, could be the boss or the job the hero hates, could be the hero's upbringing, could be the hero's family or even friends. The enemy could be the hero undermining their own resolve. Remember though that there has to be some tension, some goal, some opposition or you won't have a story.
Your hero's allies might very well include the mentor you've introduced earlier. In fact, in many cases, the mentor isn't older and wiser and special, the mentor can simply be the best friend who says the right thing at the right time. So when you create your mentor, think about whether you want him or her or it to be outside of the hero's ordinary friends or whether you want the mentor to be someone who is with the hero throughout the journey. You might think of Lord of the Rings - Frodo really has two mentors. He has Gandalf - the mentor who is outside the ordinary course of his life, who gives him great advice but also vanishes for a big part of the book (or the movie). But he also has Samwise Gamgee who gives him great advice - different advice - but sticks with him throughout. Both Gandalf and Samwise are mentors and allies, but they fulfil their roles in quite different ways.
In the case of both allies and enemies, there may be more than one so don't worry if you find your hero fighting against more than one problem or more than one enemy. Don't worry if you find that the person you thought was a friend turns out to be an enemy or vice versa. It happens all the time and it often makes the story more interesting. Because people aren't always what they seem and we as readers totally understand that. We also understand that people change their positions based on circumstances so if your friend falls in love with your fiance, no surprise that they turn around and become your enemy.
This is one of my favorite parts of writing the book. I love writing the allies and enemies. I love writing secondary characters - there's less pressure on them to perform. They can relax and enjoy themselves and so can you.