You need to create a custom service that uses SQL to pull data from the relational database and updates the JCR. As Gaurav suggests - the details of how you do this depend on how much data. For example - if you want to move data from a single table to the JCR, you would need to read the data from the table and then decide where in the JCR you want to place the data. You could for example, create a new node at a specific location for each database record.
ALso - assume you want to pull data from MySQL. You can use a DataSourcePool to help you setup a connection. See this article for information about creating a custom AEM service that uses a DataSourcePool to set a connection to MySQL:
I'm not sure why would you want to do that. In any case, you need to understand your requirements/use cases in depth and pros/cons (essentially relational database vs jcr features) of what you plan to do. There is no automated tool available to simply lift and land database into jcr.
You'd need to identify "detailed" requirements, create a design, data model, taxonomy, site structure and other aspects and follow the sample implementation as mentioned by Scott and create a full-fledged app for same.