it sounds as if your alternator is on the way out, or it's drive belt is loose. The simplest check first- make sure the alternator's belt is tight: i.e. can only be deflected under firm pressure.
If that checks out o.k. then you need to check if the alternator is charging properly. Easy if you have a multitester: set it to the 0-20v dc range, and put it across the battery terminals. You should see around 12 volts. Start the engine- the reading should now be around 14.5 volts, increasing slightly with revving. If it's much below 14 volts or doesn't move from 12 volts then the alternator is faulty.
If that is the case, then it's most likely that you need new alternator brushes, which aren't too expensive, but you'll need to remove them to check. If you can't, a good auto electrician should be able to sort it for you.