If you are clever enough to ascertain that the alternator is not doing its job, then the rest is academic. Charge your battery, and take it to your nearest qualified bike shop or auto-electrician and get then to check it out, just to be certain. From there you can make an educated decision as to what you do, ie, if it is the alternator, then your choice is, to replace it with one from the wreckers, a re-wired one (generally best value) or a brand new one.