Select * from table1 Inner join table 2 on
table1.FirstID = table2.FirstID
That is the syntax regarding on how to use Inner Join must have a primary key or a foreign key
Basically im proposing something to you.,,
If your using SQL server it is better to put in your SQL statement and try to execute it in new query of your database. In that way you can easily see what is the error of your SQL command.
Happy coding