I have a few circuit breakers in my house that, between outlets, lights, ceiling fans, and vent blowers, have as many as 18 items on a single circuit. I know that's too many. Question is, how can I tell where to disconnect and run a new line to the circuit breaker box in order to split the load in half or in thirds?

You need to determine exactly what items are on the circuit and how many total watts or Amps are used. Lights are easy, watts, then watts/volts=Amps. Vent fans are most likely Amps. Then do a circuit layout of all the items and determine where to break the circuit. You will need a junction box, easily accessed, where you can tie the new line in. Some people install a double Old Work box but install only 1 outlet and blank out the other side to get the space for the connection. Make sure your new line and breaker is of the proper ampacity.

In doing the circuit layout, are there any tools to help figure out how the wiring runs from item to item? Is the outlet on that wall before or after the one in the next room. Each of the circuits go into two or three rooms.

Hard to say not knowing how your house was wired. You can buy some testers but they only tell you that power exists. I have never seen a circuit that loops back and forth from one room to another. If you feel comfortable doing this, determine the circuit layout, turn off the power and then disconnect 1 side of a duplex outlet that is about half way in the circuit. Wire nut the cold side wires and then turn on the power, you will have power to some but not others. Depending on the load, you could possibly start your new circuit there.