Someplace means somewhere — in or to a place you are not naming, either because you don't know it or because it doesn't matter. 'Let's go someplace quiet.' 'I left my keys someplace in the kitchen.' The meaning is exactly the same as somewhere; what differs is the flavour. Someplace is American and it is casual — you hear it in conversation and in dialogue in films, and you will hardly ever see it in a formal essay, a report, or a piece of British writing. British speakers understand it perfectly but almost never say it; some of them find it slightly odd. So the safe advice for a student is: understand someplace when you hear it, and write somewhere. There is one more thing worth noticing. Someplace and somewhere are single words, but 'some place' as two words is ordinary English everywhere: 'there must be some place we can sit down' — there, 'place' is just the noun 'place', and 'some' is describing it. Both are correct; they simply aren't the same construction. The same American pattern gives you anyplace, everyplace and noplace, all of which are casual in the same way, and all of which have a safer -where twin.