Equipment- All you will really need, is the light, the tank, some salt, a silter of some sort (if this is the specv the AIO section will be your filter), and probably another small pump for flow, like an aqueon 600 or similar. Probably won't need a heater because the pumps will keep the water warm already. Need a refractometer. You can make it simple, or expensive and complicated. I'd recommend keeping simple to start and you can upgrade equipment as you read more, if you want to. If you keep everything simple and cheap you could set the tank up for probably 200 bucks.
Sand- You can use sand from Petco, but I'd get the "caribsea" bagged sand, or for a cheaper option, the bagged aragonite. Not just the Petco brand white inert sand.
Fish- 5g is going to be too small for a clownfish for sure. They are super active. You could get a baby and start it in the 5 if you want. But if you want a permanent fish, I'd look at either small blennies like a bimaculatus; or gobies like a neon cleaner goby. A goby/pistol shrimp symbiotic combo would be cool.
coral- in a 5g with the hipargero you can do pretty much any coral. Soft coral is going to be much easier than a bunch of stony corals because the hard coral consume Mg, Alk, Calc and you may need to dose these things. You can do a couple frags of stonys without needing to dose though.
anenome- It may be hard to do an anenome because they walk around and sting everything, in a tank with not much room you would probably end up with only the anenome. You could do LPS corals with tentacles that look like anenomes, called euphyllia, that don't move around. Or if you love the anenome, could devote the tank to it.
Rocks- You'll want about 5 lbs of live rock. You can buy that at Petco as well.
Water changes- Usually once a week water changes will be fine. You'll have to top up with fresh water every couple of days to keep the water level and salinity stable. The WC's replenish what the corals use up- if you have stony corals you may need to do more frequent ones to avoid dosing. If you have mostly soft coral and a few stony, once a week about 20-30% should be fine. To mix the salt, just put your freshwater in a bucket with a pump, and drop in salt until your at 1.025 or whatever you want your level to be. Use a refractometer to measure salinity.
Hope this was helpful. More questions, just ask.
