How can one find Polaris?
In which constellation does Polaris lie?

Find Polaris by tracing a line from the pointer stars of the Big Dipper. Polaris lies in the constellation Ursa Minor, the Little Bear. As explained in Starstruck Tonight:

If you can find the Big Dipper in the sky, you have a skymark to orient yourself both on the Earth and in the Heavens. The two stars that form the pouring side of the bowl point to Polaris, the north star. Polaris is a rather faint star about five times farther away than the distance between the pointers themselves, and marks the tip of the handle of the Little Dipper. The Big and Little Dippers pour into each other, just as the Big Bear and the Little Bear ceaselessly turn around and around the northern sky. The Guard stars of the Little Dipper protect Polaris from the Great Bear, just in case he might try to catch the North Star for himself.

See circumpolar stars.

Will Polaris always be the north star?

No; see Precession!