Taking down walls

[hannahsmum]

First off I appologise for my stupid questions as I know NOTHING about diy, which is why I am hiring people to do these jobs. I'd like to know what they are saying and where to start though.
Anyways, I would like to take down a section of a retaining wall, put up a wall bisecting a room and maybe take down an additional wall.

Where do I start with this? Do I call an architect or just go straight to a builder? Can anyone tell me what the steps I take are?

 

[Static]

Putting up a new wall shouldnt be a problem for a builder as long as its studwork.
Removing a wall, will depend where the wall is and if its load bearing. If in doubt get a structural engineer in to give it the ok.
Removing a section of retaining wall.. really depends on what you have and want. If its a 3m wall retaining a motorway then NO. If its a 400mm brick wall retaining a flower bed then its a different story.

If its just a few items of change then getting an architect involved can make things very expensive.