Turn on your lights, while you watch the lights have someone try to start it.
If the light drop in brightness or go out then the battery is the culprit, consider charging it and if it still does it then get another battery.
Then of course there is the cleaning of both the battery cable ends and the posts of the battery.
Then use some de-OX, anti-oxidation, paste on the battery connections.
After that is it isn't fixed you may need to trace the battery leads to where they are going may have loose or bad connections.
The last thing you will need to,check if all else fails are the connections on the starter and then the starter itself.
Start charging, test for voltage, check connections.
Luck,