I know this thread is several months old, but it is about turtles so I have to jump in. Also, if the OP was not scared off, I hope this information can help out.
Yes, a bigger tank is needed. It is said that an adult female slider would require about 125 gallons alone. You could get away with a 55 gallon tank for now however.
I do not know about that type of filter (el-vheapo) but turtles need heavy filtration or one must change their water very often (and that becomes a pain very quickly). A strong filter and feeding in a seperate tub (out of the aquarium) is suggested.
As for toys, I have heard of various turtle keepers giving their turtles "toys". They will mess with them and some people consider that "playing". Naturally, turtles dig, climb, hunt and remain active in their day to day lives. I feel that stimulation of some sort (besides feeding) is important. Check Austin's turtle page (website) for more information about toys and other forms of enrichment.
Yes, you do need specific types of food. What are you feeding the turtle now?
Water should be treated with something like Tetra AquaSafe (or another brand).
For a slider, you can use fake plants, as your turtle grows it will start to eat real plants.
There is a lot of cool decor for turtle tanks. I like to use natural looking things like driftwood and rocks.
Do you currently have a UVA/UVB sun light? How about a basking light? Those are both very important for turtle health. You should also have a heater for the aquarium water.
Sliders can be kept with out a dormancy period. Many yellow bellied sliders stay awake all year. It's normally sliders in northern areas (that are kept in ponds) that go to sleep in the winter. The same species of turtle will remain awake in places like Florida (or when kept indoors).
I will PM you with a great care sheet from Austins Turtle Page. As with many pets, sliders don't often get optimal care because new owners have a lack of knowledge. If the owner gets overwhelmed, the turtle then gets dumped in a local pond or given away. And there are already many turtles in need of homes. If someone acted annoyed towards you, it's just because they have seen this happen too often. However, we all have made mistakes and we all have to learn. I hope the info I supplied can aid you and your yellow bellied slider.
Feel free to ask any more questions. I hope you weren't run off.
Good Luck!