Begin with lifedrawing! Try to find a class near you - they're VERY useful! If you can't find a lifedrawing class, start using photos for reference - or draw stuff around you. Draw stuff you find on your desk, or stuff that's right outside your window. In the case of learning how to draw realistically, copying is totally a legitimate way to learn. In lieu of lifedrawing, photos are your very best friend.
After that, it's practise, practise, practise! It may very well take years, but they're going to be fun and interesting years.