If your car is cranking slow, or not at all, but you still have lights. Check your connections before buying a new battery. If they are corroded your car may not crank over.
You will need a 8mm wrench, a wire brush, dielectric grease, and contact cleaner, this is not needed but will make it easier.
Start with your negative cable. This will be the first cable off, and the last cable to be put back on.
Now use your wire brush to clean all corrosion off, this is where the contact cleaner helps. do this for both cables
Now do the same on the battery post.
Be sure to clean the other end of the cable, such as at the alternator.
You will also need to follow the cable down to the starter, and where the negative cable connects to the block. Be sure to leave the negative cable disconnected from the battery during this whole process.
Apply a small amount of dielectric grease to each connection.
Tighten each connection.
This should be done 2 times a year. Once in spring and again in fall.