First of all, you need an interface.
I recommend the Velleman K8000 to start with .. you can get a PLC for it too(the K8001, powered by a PIC).
http://www.velleman.be (Can also be found at retailers .. Maplin is one of them)
If you wan't to interface anything straight to a laptop PC the easyest way to do it is using the
parallel port(like the K8000 does). The serial port is used for communication only.
The biggest problem in using a laptop PC(or any PC for that matter) to control low-lewel devices directly is the timings.
Since your laptop is running a multitasking operating system, you can never run a program in
real-time.
The best solution is to use microcontrollers for all low level operations, and simply use the
laptop PC to give orders to the MCU(microcontroller unit) over serial or parallel(or USB for
that matter..)
Here's a flowchart for one of my robots(it doesn't use a laptop at all, but a laptop could be
replaced for the "Main MCU" you can see there):
**broken link removed**
As you can see, everyting is cut down to different layers, and then the layers communicate
with the Main-MCU who makes the final call of every operation that's made.
You asked about serial communications:
Serial is (usually) basicly just a text-string that's sent out on a serial-port at some given speed.
ASCII is used the most.
You should be aware that most microcontrollers send out serial-signal where LOW is 0 and HIGH is +5VDC(varying on controllers), but the serial-port on your computer(RS-232) want's you to send it -5VDC to -15VDC for LOW and +5VDC to +15VDC for HIGH if I'm not mistaken(don't shoot me if I'm wrong, I try to use something from 7-10VDC)
There is a MAX-chip available that can make these voltages for you in an easy way.
I think it was the max-232 or something like that..
I'd recommend that you read as much about electronics as you can, and if you're not good ad
mechanics, you ought to find someone who can assist you in that area.