I bought this huge poster to put up on my wall. It’s 30’ x 40’ inches. What’s a good way to hang up to make it look nice?

Wrap it around a good canvas if you want it to 'pop'! Moderate cost but we'll worth it.

If it is printed on canvas I think it would look great wrapped like the other commenter said.

If it’s in paper I think having it framed would be a good idea too, with a thin white mat, and black frame or whatever goes with your room like this

Edit: Saw your comment saying frames are a bit expensive. Here’s one at VistaPrint for around $40. And this is one of those online stores where checking retailmenot, and other coupon sites would likely have working codes for.

Double sided tape maybe?

With the canvas note go to goodwill they often have big old paintings you can use the canvas and or frame yeah know when we can go out again

If frames are out of your price range you could look at going to a hardware store and getting lengths of half round dowel (some places will even do the angle cuts for the corners for you) and use that as a "frame" can then paint it yourself if you like, or leave it plain wood look and double sided tape the whole thing to the wall.