Please go do research on Red Eared Slider....

Your turtle would need some fiber in the diet. Give some vegie like kang kung or sawi. Sometimes a few feeder fish is good. Pellets, you must be careful coz some have too much protein. It is always good to provide some fresh food... like human lah, if we eat canned sardines everyday not nice rite? You would also like to eat freshly cooked fish once a while.... so get some fresh vegies or feeder fish for a change. You can still continue feeding the pellets.

You can scrub your turtle's shell with an old toothbrush. Gently scrub off... do not use soap and apply little pressure.

Yes, that is ok. It needs to have a rock above the water so when it wants dry land, it can be on the rock.

Yes, you have to sunbath your turtle. Good to have 10 to 20 minutes of natural sun light everyday. Since they are cold blooded, they do need heat so a normal basking lamp is good to give some heat. The sunbath will provide the ample amount of UV rays and produce Vit D3 but a basking lamp will only give heat.

my tortoise being too hyperactive when he saw me?
is he healthy? curious about it

as with almost all american emydidae (sliders, cooters, maps, etc), res's shell will lose their attractive markings and bright green color, becoming less pretty as they grow.

this is their blessing and also their curse... people gets attracted by their bright color and take them as pets when they are small,only to become bored when those res turns into big unattractive turtles.
spend 5 digits and you won't have to wait 

care to share some pics?