Great Weapon master is honestly sub-par. I have it and I rarely find myeself using it or it being helpful. On paper it looks great but in practice not so much.
I'll start with some questions;
What role do you want to play in combat?
What are your stats/how are you getting them(rolled,array,point-buy)
What do you want to do outside combat?
What does the rest of your party look like?
As for general advice here's a few things I wish I had known about polearm fighters when I started mine over a year ago.
1. You are not the best candidate for the tank.
----Not having a shield and not being able to use Opp attacks within 5' makes it hard for you to protect others well as once people enter your 10' reach they can go wherever they please. Sentinel helps with this but it doesn't give you as much control as you normally would have.
2. You are not tough enough to stay on the front line.
---Prepare to get hit and go down A LOT. I have +1 plate and a splash in barbarian to be able to rage and I still get hit all the time and drop pretty often. Given my party doesn't have a real tank and my con is low(another mistake).
3. Don't neglect your Constitution score.
---Starting as dwarf will definitely help you here. But in general remember that you need to be alive to deal damage. Trust me that 20 str is tempting but if you still have a 13 con you aren't going to be very effective into the mid-game.
4. Tripping attack is overrated.
---Sure it gives you advantage and can be very nice for hits 2-4 but it doesn't help you control the field. Pushing, Commanders, Goading, and Menacing are far better. I do use tripping often but it wouldn't be my first pick if I could go back. Remember that being prone gives disadvantage to your ranged allies, which can make focus-firing difficult.
5. Don't waste all your dice on Riposte.
---Riposte is a great maneuver and can up your damage output considerably. That said you only have a few dice per long rest and it doesn't give you any added benefit like pushing or goading. I suggest only riposte-ing when you think you are close to killing your target/ending the combat.
6.Commander's Strike is awesome.
---I know I know, your bonus action and one attack is a steep cost. But if you have a ranged rogue or a ranger(or both) in your party they can use sneak attack/colossus slayer on their reaction, hitting massively harder than you would have in those 2 hits. I have a ranger in my group and on an average commander's strike he hits for 30+(we are near 20). It is well worth it.
