Looks like you are in the same stage as I am right now!
My story goes the same as yours and my skill is nearly just the same as yours at drawing and I'd like to do real life portraits one day too.
Not many suggestions from my part, but I can tell you what helped me go forward:
1.) The very fact that you are thinking about your artistic skills is a good start, it means it really means something to you. So, you have a very high chance of succeeding.
2.) What you drew at the age of 14 is WAAYYY better than what I could have ever drawn at that age. So, dude man! You've got talent! You just have to work hard on it and keep it up!
3.) Easy to start: Buy good tools first or start with something small.
4.) Looks like you have found your inspiration already (It's that portrait you want to be able to draw one day)
5.) Why not take tutorials from youtube and try drawing some of the portraits you want to? It's never too late nor early to start. Try drawing that portrait that you want to draw, now itself. Take tutorials from youtube
6.) It's youtube that helped me big time. So, I suggest you take help from it, it gives you step by step tutorials and basic information on art from which you can start your own experiments and journey in art.
Here's what helped me: http://www.youtube.com/watch?v=KSukCxg1jxQ
Few things to keep in mind:
1.) Shadows and lighting to give the picture its depth
2.) Proper shades of colours
3.) Using different shades of the same colour to add detail and depth, and the proper selection of colours and their hues to show the proper lighting effect
4.) Pay attention to the minute most details
6.) Even sketching a little eye can take a day if you are going to pay attention to that many details that will bring perfection to it
7.) Keep practising and learning. Lessons in art are never ending and one can never really be a pro artist, there's always more to learn.
8.) Take help from seniors. Youtube helps.
Lastly dude, love and enjoy what you are doing!
Wish you all the best from my side!