Your child has just discovered his own autonomy and is extremely curious about and excited by the world.
You have to come up with ways to stay right on top of him and as much as possible, distract him from his intent with something more fun or interesting. But in cases where he won't be distracted, you have to keep saying no, explaining simply, and redirecting him even when he screams at you.
In general, "Look what I have!" and other exciting and interesting distractions will make your life easier if you can manage to have a few in reserve.
I agree with the person who said you need baby gates over your doorway/ledges.
For some situations you can try one of those toddler leashes if you're worried about losing him in a crowd or the relative level of danger surrounding him being extremely high.
But unfortunately you've reached the stage where you just can't take your eyes off of him at all, and are going to have to adapt your life to some extent to work around that.