How do you know the number of seats you are filling?
I have a size of 1500. Do I just 1500 x 30 = 45k?

and if I'm not filling my stadium what do I have to do. I've been winning every game.

From the FAQ:

Attendance
Attendance is calculated by using the basic attendance (60.000) and manipulating that number by different factors that effect the willingness of spectators to come and see the game:

- League level

The basic attendance is lowered by 6.000 for each level below the first; 54.000 for second level, 48.000 for third, 42.000 for fourth, and so on until the basic attendance is 6.000. The basic attendance stays 6.000 after the tenth level.
- Team Form
Team form plays a big factor in the minds of the spectators. They are much more willing to come and root for their team if the team has a good league run and less willing to come when the team hits a bad streak.
- Team Postion
For every position above the last a team gets 1% more attendance. (For Example: a Team that is fifth will get 5% more attendance - calculated with the basic attendance, not the attendance modified by the team form or weather)
- Weather
Weather can make a big difference when it comes to attendance. If it's sunny then the crowds are more willing to go and see the match. If the weather is bad then the people tend to stay at home rather than go to the game.