Songwriters all have different processes of writing and creating. Everybody has a different way of thinking through what they want their song to sound like. Although it can be tough, a good place to start would be by picking the main topic that you want your song to be about. If you're not sure yet, then take a piece of paper and brainstorm. Come up with a few ideas and pick the one that resonates with you the most. Writing about things you're passionate about always produces the best work.
When you're writing lyrics, you never want to pretend to be someone you're not. People will be able to tell that the song doesn't come from an authentic place. The things that you are writing a song about should come from your own life experiences. It should be something that you feel strongly about. When you connect to the material that you are creating, other people will take notice and connect to it as well. It's important to be as authentic as possible in your work.
Sometimes you're going to have 15 minutes of pure genius and write a song that you love. And sometimes it's not going to happen that fast. If you feel yourself getting frustrated with the song you're working on, take a moment and walk away from the project. Sitting there and overthinking is not going to serve you well. It will only make your writer's block worse. Sometimes all you need is a break or a good night's sleep. Let your mind recharge and come back to it later with a clear head.
Sometimes, writing music can be frustrating without someone to turn to for help. If you feel stuck, try reaching out to other writers for advice. Collaborating with other songwriters can be a great way to produce a project that gains traction. It's also interesting to have different perspectives working on the same piece. It may help you look at songwriting, and music in general, in a completely different light. Build a group of writers and get to work!