no memory left on iphone 5s

hello, firstly i'd like to say thanks in advance to anyone who can help me.

i have an iphone 5s 16gb and im struggling with the memory on it now (have had it since launch) for about a month its been telling me i have no memory left atal but i have so little apps and music on my phones theres no way this can be the case. i have been looking online and it seems as though its old ios software taking up my memory, if this is the case does anyone know how i can remove them and explain it in quite simple terms as i'm not the best on computers, (i am using a windows pc and not a mac if that makes any difference)

if anyone could help with this or has any other methods to free up more space that would be greatly appreciated!!