How to determine which walls are load bearing?
First, you must determine if the wall is load-bearing or not. As long as the wall you intend to remove is not load-bearing, you can take it down with little thought toward structural support of the ceiling above. But for load-bearing walls, you will need to carry the weight of the level above by other means, such as constructing a beam or buying a special laminated beam.
It is important to remember, though, that while the beam carries the load of the ceiling above, all that load is then transferred at the ends to vertical post structures, created by a paired construction of king studs and jack studs. Thus, the floor below those jack studs needs to also be strong enough to the task of carrying the entire load of the removed wall. Normally, this is not a problem, since the original load-bearing wall likely was built over an underlying beam or foundation structure below the floor. But in rare instances, it may be necessary to add some form of additional support under the bottoms of the jack studs. At the very least, the jack studs should be positioned directly over floor joists to support the weight. When planning a wall removal, it is always best to consult a builder or structural engineer for advice on the size of beams and the size and location of jack studs or posts.
Plan on spending a full week on a project of this complexity. It is likely you'll need at least five eight-hour days of labor when you include the wall and ceiling repair, as well as the final cleanup.