MacBook Pro takes literally 1 hour to start up! Help

My mac was running incredibly slow and after rebooting it, it takes OVER AN HOUR to start!

It's stuck on the white screen with the apple and loading bar half way across.

When it finally started up I can't even press anything with it crashing/not responding or having that annoying colourful cursor appear.

Please bare in mind I am not a whiz at computers and it took some time for me to adapt from PC to mac.

Please help!