To start writing a solo leveling novel, you need to think about the leveling system. Will it be based on experience points, completing quests or something more unique? Also, plot out the early challenges the character will face. These challenges should be both difficult enough to be interesting but not so hard that the character can't overcome them at their starting level.
Begin by brainstorming ideas for the protagonist's abilities. These abilities should be something that can develop and grow as the character levels up. For instance, if the character has a magic ability at the start, think about how it can become more powerful and diverse as they gain levels. You also need to plan the pacing of the story. A good solo leveling novel should have a balance between action - packed leveling moments and slower character - development parts.