Google says that the tank you are talking about is about 7 US gallons, correct?
If this is your first tank, it will be very difficult to maintain your water quality in such a small amount of water.
If you do decide to go for it and turn it into a marine tank, you will be very limited to what fish/inverts you would be able to add to the tank.
You might be able to get ONE very small Goby species in there with a few hermit crabs and some zoanthids.
A bigger tank would make things A LOT easier on yourself
Nick D.