Just start drawing and experimenting. Try different subjects, themes, and techniques. Over time, you'll notice what feels most natural and unique to you.
One way is to research publishers that focus on your genre and submit your manuscript to them. You can also attend literary conferences and network with industry professionals.
Post on social media platforms specifically dedicated to art and manga. Describe your requirements clearly and wait for interested artists to reach out. Or, contact art schools and see if they can connect you with graduating students who are looking for projects like yours.
Try attending comic conventions or art events. You can meet artists in person, see their work, and have a chat to see if they'd be a good fit for your comic. Social media can also be helpful - search for relevant hashtags and connect with artists that way.
You can try observing people around you. Their behaviors, relationships, and dreams could be the basis for a great novel subject. Another option is to explore current social issues or historical events and imagine fictional characters and stories related to them.