I did a lot of reasearch on Relays, Transistors and Arduino GPIO, and I also followed some schematics on other sites. I put my complete circuit into Multisim 11.0 and it worked, but I cannot for the life of me get it to work IRL.
The main problem is, that on the output (collector of the BC547) is a constant 5-6V (even though the arduino pin is set to switch from HIGH to LOW every 1 second), but in reality I need the 12V supplied by my external source to switch the relay.
Here is my schematic (you can ignore the upper part, It's just a AC-DC rectifier):
P.S.: I'm really new to this SE, so If I did anything wrong please tell me, I have no idea how or what details to post, and what to not post.