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Is the 18-55mm the IS lens?
If it is its a pretty decent start to be honest. Its fairly sharp and the only logical step up from it is a Tamron 17-50mm f2.8.
Personally I wouldn't go investing in 50mm just yet. Learn what type of portraits / landscapes you like to shoot and you may find that a 50mm is not what you want.
Decent results can be achieved with the 18-55mm IS.
For a zoom lens I'd say get a Canon 55-250mm IS. Very sharp considering its price and dont be scared to go 2nd hand for one, they can be had for around £130.
This imo. If you're likely to want to progress with your photography then I'd say it's worth getting used to changing lenses, rather than having a 'one size fits all' in a 18-200. The more you learn about lens optics, the more you realise that 10x zoom isn't likely to be able to offer great quality throughout its zoom range and f-stops.