Trim your fretting hand's fingernails to promote the growth of calluses. Cut your fingernails as short as you can comfortably with nail clippers or manicure scissors. This will protect your nails from getting in the way of your playing and will also allow your fingertips to make full contact with the strings, allowing calluses to form more quickly. Also, wear plastic-free tuning keys to encourage your fingers to grow strong enough to handle the strings. These are available from guitar shops and some music stores. Alternatively, you could use a bow or someone else's tuning key. It's best not to tune your own guitar if you can help it! Finally, spend some time each day practicing your fretboard technique. This will help your hands learn how to move efficiently over the strings and will also prevent them from cramping up due to improper practice habits. The more you practice, the faster your fingers will become accustomed to playing the guitar. And with a little patience, you should soon find that they have become hard enough for the job.