I am wanting to add a new room in the attic, it has 2x4 rafters with supports coming down to the joists. How do I re support it so I can remove the ones coming down so I can add a room up there?
To open up the interior space you will need to sister larger dimension lumber (2x8's, 2x10's 2x12's) to the top cord of the truss system. Then you can remove the interior bracing, add larger sized floor framing etc.
That's the short story.
The longer version is that this is not a DIY project. You'll need someone with some structural experience to do this correctly. If you have a code department in your area, this will require permits. Find a remodeling contractor and go from there.
Add to Jaybee's note, it sounds like you have roof trusses not a rafter and joist roof structure. This is definitely beyond most DIYers ability and time to get it done. If your existing roof structure doesn't have the clearance you need to build the room you'll need to do a complete tear off, which requires a skilled framing crew to get the house back under roof quickly to prevent rain damage to the house. You'll also need to have the lower walls and foundation inspected to make sure they can handle the additional load of a another finished floor above.
Any advice given here is general in nature and is not necessarily valid for your given area. If in doubt check with your local codes enforcement department for what is required when doing electrical, plumbing or structural work on your house. Permits may or may not be required in your area and home owners may not be able to DIY some tasks. I have no way of knowing if you have the skills needed to complete the tasks you are asking about, when in doubt seek professional assistance.
My advice may be worth exactly what you pay me for it. :-) For the record I did not stay in a Holiday Inn Express last night.
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