Prepare Top Planks
Before starting, you need to be totally sure the wood is dry. Material that has sat on a job site will likely have high interior moisture levels from rain. If the material immediately came out of a conditioned space, chances are it is dry enough to use. Remember that if you are using floor joists, they existed in a space subject to high moisture levels (crawlspaces). When you move moist wood to inside your house, it will dry out. This is not something you want after the table is done. The best way to dry your reclaimed wood is with a kiln. A local mill might be willing to dry the wood for you. Another option is to store the wood inside during the winter. In a house with forced-air heating, the air movement, low winter humidity and heat will dry the wood for you. If time is not on your side and you are not confident in the dryness of the wood, it can be stacked in a hot dry place with 1" spacers between layers.
Next, depending on the condition of the wood and type of finish you want for the top, sanding the board may be a viable option. However, there is a good chance that, due to moisture and time, your planks will be slightly cupped or twisted. The quickest way to correct this is to use a tabletop planer. Though not incredibly expensive, it's not a common tool in most garages. Chances are you know someone you could borrow this from, or a local millwork shop might be willing to plane them down for about $20*.
If doing this yourself with a tabletop planer, start by checking for nails and excess dirt. Then rough cut the boards long to your finished dimension. Final cutting will be done after the planks are joined, so leave them at least 1" long. Run them though the planer, taking a small amount off each side at a time.
You will need to manually adjust the depth of cut while the board is going through. This is important since the board will not be consistent in thickness (or twist) over its length. Damage can be done to you, the board or the planer if you do not manually adjust it for a small cut at all times. Keep running the boards though until the blade has slightly touched all surfaces. Do not worry if the boards are slightly different thickness (less than 1/8" difference), we'll fix that below. Next, run the boards through a table saw so that each side is parallel. Only take off enough that the blade cuts along the entire edge. Each plank does not need to be the exact same width, only a consistent width along its length.
Due to the possibility of concealed nails and/or embedded dirt, some shops will decline this job due to the likelihood of dulling their blades. Be extra sure to pull all nails and punch through any with broken-off heads. Small tabletop planers have less expensive replacement blades. If all options fail, a wide belt hand-held belt sander will work after joining the planks (below). Run the sander with the belt perpendicular to the plank joints and move it consistently back and forth parallel to the joint.