Just to clarify something which appears unclear. Twin shares are generally private rooms booked by two people travelling together. If you arrive and book a twin share it generally doesn't mean you share with a stranger.
In South America at least all the 3 bed rooms me and the better half booked we had to ourselves unless they were busy. Only paid for the 2 beds aswell.
In two of the six dorms I stayed in...I had them all to myself.
Don't book ahead b4 leaving home. Adventure. Learn something.

I should have mentioned I'm doing a Top Deck tour. I understand that this forum tends to look down on tours but I would feel more comfortable doing things with a tour even if it is rushed. I decided to go with the dorm room option despite my apprehensions. It would be more of a party atmosphere which isn't exactly my scene but if I do the twin share option I'm more likely to be stuck with someone I don't get along with. I like the idea of using towels or sarongs to create the illusion of privacy.
Broaden your mind. Its why we travel..The important thing is the going.
If brits or Aussies are aboard, hang with them. YHewar 'em out. Yanks just talk about home.
"A yank whiner" is an American, who has to comment all the time, on how we do whatever back home. And how its so quaint and different here.
You'll have a ball regardless.
:)
Oh, twin share option on a tour! Yes, go for that as it does sounds like a good idea. You share but not with loads of people.

I'm doing the multishare option but I will probably only share a room with 3 people at the most. I have slept in a cabin with about 4 or 5 other girls before. The multishare tour is 18 days while the twin share option is 14 days.

Are you an only child or something? You will be fine stop stressing about silly things, you will have a great time.
Try a dorm a few times at first and you will find out how it goes for you. If you are traveling in the off or shoulder season the dorms will not be full. The idea of a sarong to hang up is a good one...especially if people are turning the lights on at night. I always use ear plugs when sleeping in a dorm room. At home I have sleep issues many nights.....when traveling I sleep like a log......(no husband and kids to worry me :0) The best things about hostels is the people you get to know staying in the dorms and all the travel tips you pick up from them. And yes, it is good to wear something modest to sleep in in case you end up in a mixed (men and women) dorm.