Hi, I had a dom who really helped me in this area. If your very new at anal, the 'pushing' might be a bit worrysome if your unsure something other than what your wanting to may also come out.
Anyhow...
He instructed me the anus is nothing more than a muscle and needs to be worked as such. Begin with a butt plug. Insert it until it becomes uncomfortable holding it there squeeze your anus , then release, squeeze, release. Do this ten times, then remove the plug. Start all over again, If you do this several times a day You will find over time you can take more and more of the plug. Always for me once I get to the wider part it feels more like a sucking in action than pushing out.