My friend recently gave me his old blackberry (blackberry 8900 curve) and my sim card works fine with it. The unlock button is a bit stiff but works eventually and sometimes when the phone is on low battery all buttons apart from a few do not work and the end call (red) button behaves like the key 'n' (an n will appear onscreen). Can someone help me please?

Low battery does not cause button problerms. More it's been dropped or got wet. Not much you can do now. If your good at electronics you can try to clean the keypad after taking it apart.
search the internet for videos or the forum FAQ Disassembly Instructions
for further info on cleaning or repair.