if say, portraits and landscapes? then which lens will be suitable?
you probably need more specifics.
but if you are not sure what you want to shoot, try the kit lens, firstly it is cheap, secondly the range is good for "trying out" and while the optical quality might be inferior, on the bright side in the future you will be happier when you pay the money for a better quality lens.
no one likes switching lenses, but sometimes you would need a variety of lenses. for portraits, at different distances, at different "cuts" of the entire person, you would need different lenses, to be honest. no one is going to use a 10mm lens to shoot a head-shoulders shot, for instance.
landscapes likewise - you could use an 18mm lens for a landscape, in other situations 70mm will be good, in other situations 200mm might be good.
understanding your own personal preference for perspective, i think is more important than stating the type of shots, because from abstracts to still life to portraits to architecture to landscapes.. all these need a variety of focal lengths and whoever tells you otherwise must be kidding themselves. but the "inner visualisation", how you see things.. that usually depends on the focal length you are best familiar with.