You need to do two things. First, go to the script editor from the 3 dots, and change the number on line 6 to the maximum number of students for your club - 1. Then from the spreadsheet, insert (or delete) however many new rows you would need and also change the range in cell A41:
EG. If you want the club to allow 30 students, change the code on line 6 to say >29, then add in 10 more rows to the register and in cell A41, change the range from B1:B21 to B1:B31, and also change the range for the waiting list in cell A43 from B22:B31 to B32:B41
To change the description, go to the form and edit the description underneath the title.
To edit the confirmation message, go to the settings cog and click presentation and edit it there.
This can be done by going to the script editor from the 3 dots, then adapting the two lines of text on lines 8 and 9 where it says form.setDescription(.... and form.setConfirmationMessage(.... (Make sure the text you want it to display is between two speech marks.)