why supply curve slope upward

Answer 1

Most supply and demand curves assume perfect competition.
An increase in Q (quantity, horizontal axis) means there is a bigger demand (in a competitive market), so price goes up.
On the other hand, when there is low demand for the commodity (Q goes to the left), suppliers tend to "dump" goods at a lower price.
All this result in a supply curve going upwards.
See, for example, further explanations at
https://en.wikipedia.org/wiki/Supply_and_demand