I got a little bored, so i searched on google Morrowind online.I found a mod that could enable that(but that isnt very important, since im not in the right section for that) , but it requires bloodmoon expansion.
1. Do you have to have installed another expansion in order to install Bloodmoon?
2. Do you start a new game, like somewhere else, with a different main quest, or just continue the quest from the original morrowind?Because i would like to make an Shaman

1. Nope. If you don't have the other expansion (Tribunal) everything will work fine. If you do have the Tribunal expansion as well you need to install that before Bloodmoon and not after or that will cause issues.
2. You continue the game as usual. Bloodmoon adds the island of Solstheim to the north west of Vvardenfell. It has a different main quest that doesn't really have anything to do with the Morrowind main quest, but you're still playing the same character. If you ask about the latest rumors anywhere, they should mention Solstheim.

Install Tribunal first (before Bloodmoon) even if you don't intend to play that expansion. If you don't it can get terribly messy. But watch out for the Dark Brotherhood.

1. If you check the bloodmoon readme, it says that it will run fine without tribunal, but if you then want to install tribunal you will have to reinstall bloodmoon.
2. The guys above answered this pretty well