To expand on what has been said, yes, it is a good idea to check the voltage at the battery but some explanation might be in order. First, charge your battery fully using a slow charging trickle charger and let it charge overnight.
OK. Start the bike. Put your multimeter in VDC scale and touch the probes to the battery terminals... POS to the positive terminal (will be red probe) and the NEG to negative terminal (black probe). At idle you should see just a tad over 12VDC. Now, rev your engine to 4K rpm. You should see 14.5VDC.
If you don't charge your battery properly on its initial charge, it may NEVER hold a charge well. The initial charge is done after filling the cells with acid filled to the upper line on the battery. The charge should be done using a bike-specific trickle charger (or one that is for small batteries) but certainly not with a car charger which puts out too much voltage and too fast. If you charged like I mentioned but have voltage issues at the voltage test; that is, too high or too low, there is a good possibility that your regulator/rectifier OR alternator are bad or you have a bad connections and this will require further trouble shooting.
Explain the initial battery charge procedure and also do the multimeter test as I described. We can get you going pretty quickly as this isn't a complicated problem.
