You've got internet (and the phone also) over the cable TV antenna wire. The easiest way to get both to work is to buy a splitter (a good one for around 10-15PLN). Now the wiring is a bit tricky due to the fact that there are so many plugs :).
The best splitters have F type sockets only also the most commonly used socket on the cable modems (screw on type). The TV's usually have an M or D socket (I don't really remember which one - D plug fits into an M plug and vice-versa).
So all you have to buy is:
-1 splitter (pl. spliter) with 1 input and 2 outputs (or 3 if you'd like to add a second TV)
-75Ohm antenna wire to connect stuff (ask for RTV antenna wire)
-enough plugs to put on the wire
Don't buy this stuff in a large store, avoid plastic casing of the plugs (metal ones are better, the plastic ones fall apart after a week). In a small, specialised store the staff will probably know how to properly put the plugs on the wires - for a small fee or for free).
Schematic attached - red is the way you used to connect the telly. Required plug types are in
[bold in square brackets]Also most cable TVs have a paragraph (in their Terms of Service) stating that they will add a free splitter free of charge - so bug them for it or ask someone that. Most cable TVs use a wall-mounted splitter with two F-type outputs.
If you've got any more questions just ask.
P.S.
The forum seems to cut the image... click to view whole.
Schematic