Safe resistance for speaker jack
If you are talking of a typical line out or headphone jack from a laptop/PC soundcard, then this is not suitable for driving speakers without some amplification.  

So assuming you are talking of an active speaker setup (i.e. it has some built in amplification) using a line out signal from the PC, you will want an input impedance of around 10k\$\Omega\$ or higher. The idea is not to load the output impedance, which is typically around 100\$\Omega\$ according to the Wiki page on line level. This maximises voltage signal transfer. 

For the wiring, it is L, R and common as you say, so connect the common to the ground of your circuit and L and R to the 2 inputs. 

EDIT 

The impedance is low on your headphones as they are not an input to an amplifier stage, rather they should be driven from the output of one. The >10k impedance I am referring to is a typical input to an amplifier stage. Since it's just signal, not power transfer that is the intention here you want to have a high input impedance so as not to load the signal and cause the voltage to sag.
The line out (say 100 ohms) will also be suitable for driving small headphones, since they require very little power. Since it's power you want to transfer to the headphones, the low impedance is better. For speakers though it's nowhere near enough without an amplifier.
Read the Wiki line level page linked to above, it explains this all pretty clearly.
For a complete understanding you might want to grab a book on basic electronics and study ohms law - this will also come in handy for the next part.  

Building an amp:

There are a million and one amplifier circuits out there, ranging from a couple of components to huge complex power hungry monsters.
Since you are new to this, I recommend starting with something that does as much as possible for you. Luckily there are cheap ICs that do just this, just needing a few simple connections to make a reasonable quality amplifier.
There are many to choose from (just go on any large component vendor and type "amplifier ic" and you will get loads of options) so it could be a bit confusing - here's one option:  

The TDA1517 



This is a 2 x 6W (dual output) class B integrated amplifier. I only picked this one as I have used it before and know it to be a "solid", no fuss amp. If you need more power or lower THD+n then have a look around on Digikey, Mouser, Farnell, etc. The connections are pretty simple:



So you would connect your L (probably tip) and R (probably ring) to "input 1" (pin 1) and "input 2" (pin 9) then your common (shield) to pins 2 and 5
(there are two pins for signal/power ground so you can separate larger power currents from small signal currents and keep things quieter - put your power on the other side from your input so the power current doesn't share any of the signal return. If this seems confusing, don't worry, just connect pins 2 and 5 to ground and it will probably work okay even if you don't lay it out perfect)