Well, let's see here. Considering your tank is a five gallon, I think it would do best with one smaller fish and perhaps a couple inverts. People suggest something like a firefish goby, chromis, or a damsel for a tank of that size. Keep in mind I have never personally owned any of these fish, but in my opinion, I would go with the goby. I've heard they have quite a bit of personality and are known for not moving around too much in a tank, making it a great candidate for a 5 gallon. You can see the firefish goby here:
https://www.saltwaterfish.com/site_11...ot_parent_id=4
As for invertebrates, I would go with a scarlet leg crab and a snail.
Here is a basic list of items you will need for a new tank:
1.) Thermometer
2.) Hydrometer
3.) Test kits (nitrate, nitrite, ammonia, and pH for starters)
4.) Water supplement (I suggest Kordom Amquel Plus)
5.) Powerhead
6.) Power filter
7.) Hood
8.) Light(s)
9.) Live rock and/or other decor
10.) Substrate (sand is recommended)
11.) Heater
Of course, there are other obvious things like salt mix, a net, etc. Keep in mind that with a tank of that size, it will need to be closely monitored. Any small change in the water parameters could hurt your inhabitants. Hope this helps!
-lefty