Getting an android smartphone is not a difficult thing now a days, but maintaining an android phone in good performance mode is a bit difficult thing. The most important problem most users facing now is the slowing down of the phone.
Now a days the android phones are getting various updates via system updates and other application updates frequently and thus will reduce the internal memory soon. It was one of the main problem regarding all the android phone. Internal memory is very important for the smooth performance of the phone.
Maintaining space in phone is very important, try to keep enough space in the mobile, keep only required application only in your phone. Remove unwanted application from the phone, also use some junk cleaning application like Clean Master, to remove junk files, it will remove the unwanted files from the phone, it will also remove the residing files of the all the previously uninstalled application also. Thus you cab save some amount of space.
Also uninstall unwanted application, because the installed application will always runs in the background and will take memory and it will always connected to internet and it will cause battery and memory drain.
Its always better to buy a smartphone with enough internal memory, preferably 16GB or minimum of 8GB, because now a days most applications are of 10-20MB of size and will take more memory with data.