If you have data files (e.g. pictures, videos, music) in system memory, you could get a file manager from the Play Store (I use a free one called File Manager) and use that to move the files.
Not sure if that's going to be the issue, though ..
More recent stock versions of Android no longer have the split between system memory and internal storage, however it is maintained on older phones as merging the two partitions would erase all data on the phone.
You may be able to backup everything, root your phone and re-partition the internal storage - and optionally upgrade Android - so you no longer have the issue. See the rooting sub-forum for your phone for further information.