I would try to lead by example. Just because you came to some practices, regardless of how good you are, doesn’t mean you’re going to get people to listen or follow you immediately and it especially doesn’t mean the coach will buy in to all your ideas right out of the gate. You need to lead by example. If there’s some dead time, which there always is, see if you can’t put some more work in on the side and get a few of the guys to do it with you, whatever it is, be that flat ground work, drills, sprints, etc. Start small and just show that you’re hungry to get better and make your teammates better. People will respect that and follow that behavior in time and through that you can start changing the culture of your team.