One of the most important parts of developing a project is the necessity to have a clear picture in mind about the end goal. Namely, knowing the target audience that this project has and the features that it will include. This means that we need to make sure we are informed as much as possible about the business logic, and then be able to do the implementation of all the features as needed.
DBDesigner is a really great tool when it comes to creating database schemas for your application. It gives you the opportunity to create any number of tables that you want (as far as I am concerned). You can add any data type attribute that you want to any table that you have created, and also have certain attributes serve as foreign keys. This way, when you set up primary keys and foreign keys respectively, you can even see relations between tables of the database that you are trying to create.
You can use your email and create many projects, and get back to them whenever you want. You can also invite your colleagues via email and expect them to collaborate with you in the preparation of that schema.
When you are done having at least an initial version of your database schema, you can then export it as an SQL script for the following database technologies: PostgreSQL, SQLite, MySQL, MSSql and Oracle.
DBDesigner has other features as well, and would definitely recommend you to give it a try.
I heard about this tool when I was doing a pair-programming with a colleague and have found it really helpful. I hope you benefit from it as well.