hellophant:
I didn't understand. Common cathode (=negative pin, the one that should be grounded?)I need to look up what multiplexing is..
If I have potentiometer, can I let that one act as a resistor?
Arduino ---> Potentiometer ---> All 7 LED's?
NO.
Using one series resistor on the common pin will give a constant amount of current that will be divided between all the LEDs. Their brightness will depend on how many are turned on at once. It looks bad for 1 to be significantly brighter than 8. If you want a consistent brightness for all segments, each segment needs it's own resistor.
On top of that, LEDs can be roughly approximated by a constant voltage load, and it's not a good idea to put constant voltage loads in parallel.