Can a sith marauder wear helmets or heavy armor??

Helmets: Yes. Heavy Armor: NO. Medium armor can look pretty elaborate but if the Armor is classified as Heavy armor you can't wear it. i.e. Heavy armor helm: no, medium armor helm: yes

What are the reasons people play pvp? Should I wait till I'm a high level to try pvp?

If you're talking about why people play PvP in general is because of the competition. Beating people can be more satisfying then Com Controlled bots. You can start PvP as soon as you hit level 10/leave starter world and it's up to you when you feel like starting.

How do I send my companions on crew missions? I heard you can send them to go find certain things.

Once you have crew members you buy/train skills for them. These can be found in most market areas. Look through them and see which ones you want.