ACL means a lot of different things
Sign up for the C3 presale email list at the top of this site: https://www.c3concerts.com/. That includes most shows that take place at the venue ACL Live aka Moody Theater. Not all shows there do presales. You may want to consider their venue specific list as well, http://acl-live.com/venue/mailing-list
If you are talking about the PBS tapings for the show Austin City Limits, there are no (besides sponsorship) ways to buy tickets. You sign up for individual tapings at http://acltv.com/upcoming-tapings/ and hope they email you a ticket. If you do not get a ticket in advance via their drawing, you can line up ahead of time and at 7:45pm they will let the standby line in if there are seats available. For popular shows, line up early and understand that they may not let anyone in from the standby line.