Some of the easiest plants are the swords. I'm not a plant expert by any means, others here have much more success with the high-tech, super colorful stuff. Some swords grow well in a variety of light conditions, are pretty forgiving of water parameters and, depending on the variety and lighting, can have very nice color.
Can you give more info on your lighting? Is it a T12, T8 or T5HO?
Single or dual bulb?
I recommend various swords including the old standby Amazon sword, the Keiner Bar (nice red leaves), Ozelot, Red Flame, or (for smaller sword) the Rosette.
Stay away from microswords, they need lots of light and grow very slowly.
Other easy plants, somewhat depending on your water parameters might include the smaller Vals, Green Cabomba, Elodea/Anacharis/Egeria densa (a variety of names for the same thing), floaters such as Frogbit or "Dwarf Water Lettuce.
Others will add suggestions, you have lots of options. The only other ting you'll need are a liquid fertilizer (Flourish Comprehensive Fertilizer) and if you get swords, some root tablet fertilizer.
Note: if you get a sword or two, leave some room for growth, they can get big!