"I will buy the UV bulb today! At what distance should I place it?"
That depends. First, on which type you buy. The commonly available bulb are either full spectrum or heat emitting. Most flourescent bulbs for herps are completely full spectrum with both UVA and UVB. The Zoomed 5.0, 8.0, and 10.0 are my flo. bulbs of choice, though there are a handful of others which are probably just as good. Read the directions on the package...most need to be within around 12 inches from the turtle. These aren't basking lights, though, as they don't emit enough heat. So, in addition to the flo. bulb, you should buy a basking spot bulb.
Second, the distance is related to temperature. Keep bulbs at a distance where the spot reaches around 90 degrees.
OR, buy a heat and full spectrum UV bulb all in one with the Zoomed Powersun or T-Rex Active UV. They project UV up to 6 feet, so distance is related to temperature only. You'd be fine with the 100 watt bulbs. They're around $40, but they're well worth the money!! Many pet stores aren't carrying them yet, so you may need to order it online. It's a piece of cake.
"I read some info about shoftshells when I got it, I am not really a turtle guy, I am into snakes...he was doing so well since I got him as a present for my girlfriend at a reptile-show last year."
I'd recommend reading as much as you possibly can about Spiny Softshells...just type "Spiny Softshell turtle" into your web browswer. You'll get nearly a dozen care sheets, as well as natural history. Both would help you out. We can't possibly tell you all of the information that you should know in the time that most of us have on this forum.
"He is a Spiny Soft-Shell."
I asked because some are easier than others. Spiny Softies are about average for a softie.
"He is approx. 1-year old and he has double its size in the last year. He was defecating fine...to tell you the truth I do not know if he is doing it now."
It sounds like the turtle has been doing fairly well, which makes me think that this is most likely seasonal. The UV issue can take years to affect the turtle, so I'd get that addressed regardless. One thing that can happen with young aquatic turtles...Sometimes, they'll grow like weeds and eat everything in sight and then suddenly stop for no apparent reason. This can often be related to the size of the enclosure. A larger turtle produces more waste, and your softie may be fouling its water and raising the level of the toxic chemicals that naturally build up. It reaches a certain threshold, and the turtle gets ill and loses its appetite. Sometimes, they'll also get shell/skin infections. It may be a good idea to increase either the size of the cage, the size of the filter, or both. Water testing is always a good idea, and it's painfully easy. Just something to think about. Most softies need VERY CLEAN water!
"What is LETHARGIC?"
Lazy, sluggish, and just general sick-looking.
One more thing, softies LOVE crayfish. If you can get your hands on some small crayfish...stick them in the freezer for 24 hours, thaw them in hot water, and see if that sparks some interest. You can do the same thing with "ghost shrimp", which you can buy at nearly any fish store. Freezing first kills the parasites/parasite eggs that are extremely common in these crustaceans.
Good luck and please keep us updated. Also, I'd love to see a picture.