I am planning to buy a Canon 600D with 18-55 Kit lens. This is going to be my first DSLR. I can afford a 55-250 mm IS II Lens ( as Combo Kit ) or a 50mm F/1.8 Prime lens with my budget.
Please suggest which lens should I buy?
Which one will help me learn photography and satisfy most of the general purpose photography needs of a beginner?
Please note that with my current financial position, I may not be able to buy any more lenses at least for a year.
We have a lot of similar questions on the site already, such as this one, although that doesn't mention the longer zoom. The answer here really depends on what you want to do, which is a bit hard because you are just starting out. There is no one right "general purpose" answer, because what people do generally varies so much. Both lenses give you a different kind of versatility — the 55-250 for its telephoto reach and obvious zoom range, while the 50mm gives you very high image quality for the price and a wide aperture.
I personally don't own any lenses beyond 135mm, and don't miss them. (In fact, the 135mm is a manual lens I rarely even use; 77mm is my typical portrait lens.) But that would probably sound crazy to someone else. So, if you have to make this decision, think about how you already take photos and what you want to do. If you like to stand far away from the action and still get up-close photos, you might start with the zoom. If, on the other hand, you like to take portraits, the 50mm will probably suit you better. (On an APS-C camera like the Canon you are considering, 50mm makes a nice portrait length.) And the 50mm f/1.8 is noticeably more useful in low light.
You may also want to consider a prime lens in the 28mm to 35mm range — this is the very versatile "normal" focal length for this camera. See Why do people recommend 50mm or other prime lenses as starting lenses for learning photography? for some discussion on 35mm vs. 50mm on APS-C. Unfortunately, I don't think Canon offers a cheap entry-level lens in this range (unlike Pentax, Sony, or Nikon). There is a 40mm f/2.8, which I would actually strongly consider — see Which lens serves best as the only lens on a cropped body for a beginner: the 40mm f2.8 or 50mm f1.8? for some views on that.
Im going to suggest something completely different.
You outgrow the kit lens pretty fast. IQ comes from getting lots of light to the sensor, and 3.5-5.6, where the F3.5 is only at the widest end is simply too restricting, especially if you want to stop down just a bit to improve the performance.
Get the body without a lens, and add a fixed aperture F2.8 standard zoom (17-50mm), from either Tamron or sigma. It's large aperture that is close to a prime gives you a lot of headroom to play around with bokeh portraits at 35-50mm and speudo macro shots, while being an allround walkaround lens for holidays and flashless party photos. I know semi pros using tamron 17-50mm F2.8 for paid wedding photography and I use the sigma version for concert photography. Its like a crop version of the 24-70mm F2.8L on fullframe, and the IQ in real-life performance is close enough at a fraction of the price. The tamron is best for wide angle shots and the sigma is best for the long end (the portrait end), which is why I use that one. Before getting it I use primes - always switching between them, but now I just need to switch when I need 85mm (canon 85mm 1.8) and 135mm (Zeiss 135mm F3.5).
And if you intend doing bird hunting or something then you can save up for the 55-250mm which I heard is pretty good for the money and it is not that expensive so it is within reach. Doing it in this way saves you some incremental improvement steps (= waste money).
It really comes down to what you want to be able to do with it. If you want to immediately be able to take much higher quality portraits (and anything at a similar 85mm equivalent zoom range) then the 50mm prime wins hands down for the major increase in image quality over both the 18-55 and the 55-250.
That said, having only an 18-55 and a 50mm lens leaves a huge range of telephoto inaccessible to you. The 55-250 gives you much fuller coverage when paired with your kit lens, but as a super telephoto, the image quality isn't going to be spectacular.
Personally, I'd probably go with the 55-250. It's very similar in concept to the 70-300 I got for my 5D Mark iii when I first got it so that I could cover the telephoto focal range (on a APS-c body, the 55-250 is very nearly equivalent to the 70-300). It will allow you to handle any situation you encounter even if the quality isn't top notch. You can then buy lenses to improve quality at your leisure in the future but won't encounter photos you lack the lens to at least capture. If you really don't plan to need much telephoto though, then it might be worth going for the 50 prime.
