Most likely your problem is a loose (or corroded) connection at the
battery terminal (or at the starter).  Clean and tighten the connections.

Although a very simple repair, batteries are very dangerous. Wear safety
glasses & gloves. If you get any battery acid on your skin immediately
rinse it off with lots of water. Battery acid will burn your flesh quickly
and if you get any in your eyes it can blind you.

Begin by disconnecting the NEGATIVE (-) terminal first.  LEAVE IT
DISCONNECTED - then with the negative terminal STILL DISCONNECTED go to
work on the positive (+) terminal. If the cable (wire) going to either
battery terminal has become frayed and is growing green crud, REPLACE IT.
When reconnecting the battery, hook up the POSITIVE (+) terminal first and
NEGATIVE (-) terminal last (for safety reasons).

Less likely is that 1 or more cells of the battery are very low on
electrolyte (sulfuric acid). This could occur if the battery has sustained
some type of physical damage (cracked or punctured) or has been
overcharged and boiled dry. You can add *distilled water* if it's simply
low, but if one or more cells appear to be completely empty then the
battery has been permanently damaged and adding water is unlikely to help.
Use water (preferably distilled water) only. ** DO NOT ADD ACID **  Do not
overfill. Fill to the top of the split ring. If you don't know what this
is, have someone show/help you.

If all cells of the battery are LOW on electrolyte or if it takes a cup (8
ounces) or more to completely refill all the cells, then I would be a
little suspect of a malfunctioning charging system (Alternator/regulator).

Work safe!