One of ten people will start having "more than friends" type of feelings, it is just part of life.
It is normal to experience these types of feelings.
There are some important key factors in how to deal with the feelings you start having.
First think, what is it about this person that is making you feel this way? Is it only once in a while that you get these feelings, or every time your with them? You have to determine if you do take the very next step, and it does not go the way you have planned, will you be able to handle it emotionally? If so, then go for it!

• Are there any signs at all, the one you are falling for might be interested?

Some times you can detect that feeling from someone.
Watch for signs, any hints that they feel the same way too.
Just be careful.
When you feel a certain way towards someone you always tend to think 'every nice thing' they do for you, is a sign.
THIS IS NOT TRUE.
Some people are just very sweet and it is their nature to do nice things.
Just a warning sign for you.

• How do you think you will feel if the feelings are NOT returned?

If you do tell this person how you feel and the feelings do not get returned, will you be ok? Remember your heart in this process.
Some people when they get hurt, will bottle up and never want to give love another chance.
THAT IS A SHAME.
Love is something wonderful, and it should be the best part of your life.
Just ask yourself ahead of time, how you think you will handle rejection, if it does come to that.
There are ways you can go about this, without putting too much of your heart in it, just yet.

• Are you ready for the next step?

Watch for signs from the one your interested in.
This is one main key to 'the next step process'.
Be careful, know how to determine what is a real sign and what is a 'nice gesture'.
If your at that point where you believe your falling for the one friend your close to, then you should know their personality pretty well.
See how they react when your around someone who acts interested in you.
If they start acting on the jealous side, I would say you have a great shot!

• How to go about letting your feelings out!

This could be the trickiest part.
It all depends on just how close you two are, and what type of person you are.
Do something that is NOT normal for you two.
Maybe make a 'special dinner', write a romantic love letter, get a card, or just ask outright for a date.
I can not make that decision for you, cause I do not know your situation.
However, do not pick something that you always do and try to turn it romantic.
You need to voice how your feeling, but in a way you can handle both sides of the fence if need be.
I wish you the very best of luck, and hope I was some help.