Also I'd like to comment that because you only have the tank, gravel etc. and -not- the major hardware yet, you have the perfect opportunity to pause and take just a little extra time that may pay off nicely in the longer run. A heater (if you find the right wattage) and thermometer would be pretty easy to get on this first quick trip, but the filter is quite important and it helps if some thought is put into it.
Since you haven't got the tank measurements yet, we'll just go by formulas (and much of this will no doubt be in the links too..) A guideline starting point for overall filter size is to calculate 5 times the volume of the tank for the turnover rate. For example, a 10g (US gallons) would come to 50 gallons per hour as our starting point "turnover" rate. Its a bit better to look for the actual "flow rate" from a manufacturer, rather than a "rated for tank size." And even then, once you've found their flow rate, you would want to go somewhat over what they say. So a filter that advertises a "60g/h flow rate" would be ideal for our 10 gallon example.
Next you need to consider the 3 most common types of filter designs: Internal, HOB and Cannister. There are still others but we'll leave them out of the discussion as too exotic. The internal stays inside the tank, thus having the advantage of no siphon tubes for input/output, but has the disadvantage of using up tank space, having water all over it when you take it out to clean and also possibly forming unexpected entrapment spaces for fish between the box and tank walls.
The HOB (Hang On Back) usually has greater capacity than the internal and doesn't take up tank space. It generally has smaller media capacity than a cannister and its open-top design is just a bit more messy for cleaning but not much. It sometimes makes more noise than the others with its little water return over the edge of the tank. This category also carries the danger of many type of proprietary media holding designs that are not needed and just meant to keep you buying a particular "cartridge" from the manufacturer.
The External Cannister is larger and sits down below the tank in a cabinet usually. It has the largest media capacity of the three, which is a major advantage usually. It is usually the quietest and it is very nice for cleaning as the input/output tubes can be shut off and the box carried to the tub/sink and nice trays of media lifted out and rinsed in tank water. Another advantage is that they offer more flexibility in power outage situations than the other two usually. The down side of cannister filters is that they are more expensive than the other ones.
So I hope I haven't overwhelmed you with details. My own 16-year-old son would not have listened to 4 words of this! Fire off some questions if you wish. The members are all quite good at answering these.
~~waterdrop~~