So you've found someone that you like and haven't quite plucked up the courage to say anything for whatever reason, most likely a fear of rejection. We've all been there. I know so many people out there who are introverted personalities who aren't sure how to approach the situation and often end up finding out that the person they like now likes someone else and they are left hurt and wondering what they should do.
In my experience, let this one go, move on. People often get into this situation and feel like they have to win that person over somehow, while it is natural to want victory and to win the "prize" it's a bit of a waste of time in this situation because their heart and mind belong to someone else, it wants for someone and you really aren't in the picture.
What you should be doing at a time like this is taking whatever energy you have and investing it in looking for someone that is free. There is a reason you are not their crush and if it's because you have never made the move you now have to deal with the consequences of not being proactive enough in the first place. That's a life lesson.
When you like someone and they are single, you really shouldn't waste any time. If you like them then there will be plenty of other people just waiting to jump into the picture and take the chance you didn't. Ask yourself how many times you tried to talk to them in the past, how many times did you talk yourself out of it, how many times did you pace around instead of starting a conversation? Sorry to say it but you lost your chance right now and take it as a life lesson that you can actually learn from.
You need to realise that it is important to be proactive so you don't make the same mistake again. It sucks but no one said life was going to be fair! Move on from this and stop thinking you can win this one. Start doing things that you love and take chances that you wouldn't usually take, and keep an eye out for the next person to catch your eye.
If you see this person go straight up to them and start talking (no, don't be Brick from Anchorman and start with I like a lamp, that's just weird) but make an effort. While it can be easy to get caught up in believing this person would have been the better person for you, it's not always true and you need to open your heart and mind to other possible romantic prospects.
Don't keep putting in precious energy and time into someone that doesn't like you back. Think about it this way, if you were in their position, how much would it hurt to know you're hurting someone but you just don't feel it back?