Well Vic, hooking it up is a personal choice based on your home/flat/apartment setup. The three common locations are: under the sink, next to a washbasin (if you have a washer/dryer in your place), or above the sink connecting to the faucet. IMO, the washbasin is the best method. You get a Wye garden hose adapter and put that in-line between your washing machine and the main pipes. Then the other side of the wye is connected to the RO supply. Usually these types of wye fittings have their own little valves built-in so that you can turn on the washer (and leave it on) and turn the RO unit on/off. Then, the other end of the RO unit will have 2 outputs, one for "waste" as miss wiggle said, this can be put in a bin for storage and use around the house, or you can put it in the drain of the washbasin. The other is the RO out which you store for your fishies.
Putting the RO unit under the sink is another method. Here you use a "saddle valve" to pierce the coldwater supply pipe to your sink and attach that to the RO supply. Here the waste is again either stored, or set to go down the drain and the RO output is stored.
Lastly, if you have to put it above the sink, you can get a faucet adaptor which screws on to the end of the faucet to supply the unit. This only works with modern faucets and is pretty unsightly, not a great option.
The GPD quoted is in a nice happy magical theory land where main water pressure is 100PSI and temperature is 75F. If you can achieve that, you will actually get the GPD rating out of the membrane. In practice though, most household water pressure is ~30PSI and in the winter, temps drop significantly. Since an RO membrane works by forcing water through the membrane which has tiny tiny pores only big enough for water molecules to go through it, anything that reduces the force will reduce the output... Lower pressure and lower temperature (creating more viscous or thicker water) will of course slow that output down
For your size tank I'd reccomend NO LESS than 75gpd and actually I'd prefer 100gpd. You don't need fancy DI filters or eleventy billion stages, you only need 3 stages (Carbon pre-filter, mechanical prefilter, and RO), the appropriate main connection (garden hose, saddle valve, or faucet connector as explained above), and most importantly a "Fush Valve" or "Flush Kit". Flushing the membrane before each use will GREATLY increase membrane life. Mine's 4 years old and still kicking. Prefilters wear out, but RO membranes last for a long time if regularly flushed
Oh and a quick edit, if you can find an RO filter with the 3 stages, flush kit, and appropriate connection cheaply off Ebay, go ahead and get it. RO membranes are stringently manufactured so they're virtually all the same at a specific GPD rating. Also, don't forget a TDS meter!
Remember that manufacutrers rate their systems at arround 25c water supply with 60psi of pressure. Mains is nearer 30psi with mebe a temperature of 10-15c in summer... That will make a massive difference on output 
i need a crash course on this.
i guess the tap is on the unit or do you mean the main tap for the sink? 
