A load bearing wall supports the weight of additional floors and roofs. A simple way to determine whether a wall is load-bearing is to look in the attic for the direction of the joists. If a wall is running parallel to them then it probably isn't load-bearing, while if it is running perpendicular to the joists it might be or should be treated as if it is. If you can't tell if a wall is load-bearing have a structural engineer look at your house's structure. Some load-bearing walls can be replaced with beams. It all depends on the weight and span of the load. All support beams are rated according to their load and span capacities, and most lumber yards can tell you what size beam you will need.
Remove everything from the room being renovated and cover the floor surface if it is in good condition and needs to be protected.
Remove all wallboard on your load-bearing wall so that you can clearly see what's in the wall. If it has an internal support post in addition to standard studs then do not remove your existing wall until you have had an engineer look at it.
Using your ladder and stud finder, find studs parallel to and at least three feet from your load-bearing wall. Mark your stud locations all the way across the span area.
Place a two-by-four stud against the ceiling, parallel to your load wall, approximately three feet from it. You may need a friend on another ladder to hold the stud up. With your nail gun, nail the stud to the ceiling in at least three to four locations spaced evenly across the span. You should be nailing into your ceiling joist through your drywall or sheetrock/plaster.
Place a two-by-four stud against the floor, parallel to your load wall and approximately three feet from it. This creates a frame for the temporary suport, which should be built from wall to wall, matching the existing load-bearing wall.
Use your tape measure to determine the distance between the ceiling and floor stud.
Cut a two-by-four slightly longer than this height to establish a tight fit.
Wedge the cut stud between the floor and ceiling boards, then tap it in with a hammer and toenail the vertical stud into the ceiling mounting and floor studs. Try not to shoot your nail into the ceiling or floor or else you will be repairing these later.
Fit the other end of your temporary wall using the same technique, installing studs every 16 inches or less. This wall will carry the load of most standard walls. You are simply duplicating the existing support wall under the same load. Install any extra post supports as needed if your engineer told you to do so.
Install your support posts for your new beam at each end of the new span. This will require removal of the end sections of your existing wall. These support posts should each be at least a four-by-six and be pre-cut to allow the beam to be installed above them. The beam should fit snugly between the top of the post supports and the joists.
Remove the remainder of your load-bearing wall and, with help, immediately install your beam on top of its support posts. Your ceiling should not move or start to sag when you remove your wall if you have prepared it correctly. Your new beam should be directly in line with where the old wall stood. Install your beam immediately to prevent joist sag from the removal of the old wall.
Toenail and/or use plate supports to secure your beam to the posts and joists. Carefully remove your temporary wall so that you minimise damage to the ceiling and floor.
This is a thinking job, so plan your steps to install your beam quickly by having your materials ready. In particular, make sure you have help to lift the beam right after you remove the wall so you don't give the old wood an opportunity to sag or cause problems. Once the beam is in place it should be sufficiently strong to replace the wall. The objective is to replace the wall with a beam that is as strong or stronger than the old wall.
Don't risk the unknown, which could cause a catastrophic failure. If you aren't absolutely certain of the load issue, hire a structural engineer. A few hundred dollars is inexpensive compared to a real failure, and he will assist you with making exactly the correct load choices.