Memory help on the iPhone 4?

I have an iPhone 4 and it keeps telling me I'm running low or out of memory. I only have 41 songs and 2 apps I've personally downloaded. (Not joking I counted.) the apps are Facebook and Instagram. Can anybody tell me what i can do to fix this? Or why those 2 things take so much memory?

My mum had something like this,
She had all her apps open? Have you closed any apps down?
Double click the home button and your open apps should appear, simple hold them down as if to delete them. This will not delete them, just close the app.