how can i fix my battery light problem on my 1.3 ford ka ?

my battery light is on has been for a few weeks now car has been running fine until last night i was off to see a friend it was dark radio was off lights were on as i was driving my lights dimmed and almost went off then seemed to boot up again i am confused as it could be the battery, a loose wire conection or the alternater, so im told, could you help me ?