Start at the battery, disconnect all of the cables and clean and grease the posts and terminals with anti-corrosion grease - a very light coat of grease will do. Move on to the ground connection at the block, and the terminals on the solenoid. Make sure all of the connections are clean, tight and greased to prevent corrosion, and that your battery is fully charged and in good condition. This is the most common cause of starting problems like you are describing.