If your house is sagging or uneven due to sinking foundations then you can use a jack to raise the house back up to level. All you're going to need is to find the location of the sagging beams and to bring them back to level with a house jack, many of which are built for hundreds of tons. Once you have the house level all that's left to do is to place concrete pillars underneath the beams to hold the raise in place.
Get underneath your house and look for the support beams that hold up the house. If the foundation is sinking then you need to jack up the sinking side. Look for bowed beams that are sinking lower than their compatriots, or places where a sagging floor above may mark a problem beam below.
Place the jack beneath the sinking beam. Jack up the beam so that the house is level. Use the other, nonsagging beams as your ideal so that you can bring all of the house's beams back into a level arrangement. Jack the beam up slowly so that it can gradually shift and readjust until level again. If multiple beams need attention, then use multiple jacks as simultaneously as possible.
Place a concrete pilon beneath the beam so that when the jack is removed the newly arrived level remains. These pilons are spacers that you need to size to fit exactly into the gap. Place the pilon at a point near the jack, centered on the support beam so that when you remove the jack the pilon will take the weight that the jack was holding up.