A territory has to apply for admission to the Union and Congress has to pass a law admitting them. Hawaii applied, the others didn't. Also, up until 1959, Alaska was a territory too, not just islands like Guam and PR.
Puerto Rico has an interesting history in this regard. They're has been a strong separatist movement in PR since before it was a US territory. The two defining incidents that come to mind: the assassination attempt on Harry Truman in 1950 and the shooting in the US House in 1954.
In November of 1950, while Truman was living in Blair House, two separatists attempted to assassinate him. In the attempt, a Washington DC police officer, Leslie Coffelt, was killed, as was one of the attackers, Griselio Torresola. Truman was uninjured and the other attacker was captured. Oscar Collazo was sentenced to death, commuted to life by Truman, then commuted to time served by Jimmy Carter and he went back to PR, where he died in 1994.
On March 1, 1954, 4 PR separatists fired 30 rounds from the balcony in the US House, wounding 5 representatives. To this day, bullet holes are still visible in the ceiling of the House chamber. The 4 were arrested, tried, convicted, and sentenced to 70 years. In 1978 and 1979, they, too, were pardoned by Jimmy Carter.
PR has had a few votes on its status, and while they've been over 50% in favor, it was only by a few points. Hawaii, on the other hand, had over 90% supporting statehood.
A good book on PR politics, history, and status is War Against All Puerto Rican by Nelson A Denis. It's a little separatist in it's views (,IMO) but it's informative.
The other commenters are correct in the process and political will that is required for this, but it also seems unlikely that any of the current territories (except Puerto Rico) have enough population to justify statehood.
Population:
Alaska had 220K population at the time they became a state and that was considered at the time something of a lower bound. Population growth being what it is, it is likely that we would consider a larger number the "new minimum".
Of course, this in all informal and any of these places COULD become a state. Delaware had a population of only 60K when they became a state, so you could argue that even American Samoa and Northern Mariana Islands are large enough historically.
Hiya, if you're looking for more info on this topic, check out this thread, which will lead to several more