Some helpful suggestion already, if this is your very first tank, I would actually suggest you find a suitable place in your house where you would like to have the tank, not too close to a window in direct sunlight (this can cause problems with algae)
Then try to get as big a tank as your budget and space in your house allows, the bigger the tank, the better.
The bigger the tank, the bigger the volume of water you have, therefore this makes control of tank water parameters (ammonia, nitrite and nitrates) more controllable compared to a small so called "beginners tank" of 20 litres or thereabouts that your Local Fish Store (LFS) probably supplies.
And more importantly you can have a few more fish!
Anywhere from 90 litres upwards would be my recommended sizes to begin from, this is only if you have the space and budget, if you can only get a slightly smaller tank then thats fine also.
Two further tips I would suggest.
1st, do not rely on what your LFS advises you, some LFS have been known to give poor advice as they generally only wants to sell their products. So has been known they will say what you want to hear to encourage sales, be wary.
2nd tip, do your research and ask on this forum first if you are ever unsure of anything.
Do have a read of the link that NeonTetra97 has given, well worth reading.
Look forward to seeing your first tank set up and pictures when you are ready!