A girl I've fancied for more than 6 years

First, I'd like to say hi to everyone. You can tell I'm new around here, so I hope everyone's doing well, and I wish you all the best with your love lives.

Second, although this issue may seem like nothing compared to what other people have experienced, it's undoubtedly the biggest thing that's affected me emotionally. Pathetic, I know. But I wanted to at least get a second opinion on this issue. Plus, I'm guessing this is a REALLY common thing that happens to a lot of guys, so I'm hoping there'll be people who are very knowledgeable on this kind of thing. I did search the forum, but I wanted something really specific to me.

So, without further ado, here's the situation:

• There's this girl in my year who, basically, I've fancied ever since I've known her. OK, that's not strictly true - we've known each other since we were around 7/8 - but I remember having feelings for her ever since I was around 11/12, maybe younger (if that even counts). But, although I'd say it's not 'real' feelings when anyone's that young, the thing is, my feelings haven't changed at all since that age. No other girl even compares. I'm 17 now, and my 18th birthday is a month away.
• I'd describe our relationship as pleasant, but not close. We say hi whenever we meet, and I can easily sit down with her and we'll have a nice conversation. She's great to talk to. A few weeks ago, we met at this local courts' open day (both her and I want to study law at uni), and we chatted for a while before she had to go, and she said she'd like to talk more later. This didn't happen, but I'm not too bothered about it - you can't take comments like that literally. More importantly, I thought it showed she was enjoying the chat, and I guess being around me.
• The closest we've ever come to being together was when we were around 14. (Again the whole 'hilariously trivial' thing may come across here). During form time at lunch, this other girl in my form randomly dared her to kiss me (on the lips). She replied "okay!", I was told to "come here" by the other girl, and she walked straight up to me (smiling massively at me) and did it. Then on valentine's day - it was soon after - she sent me a valentine's card basically saying she'd like to marry me and she loved me and she sent tons of kisses, etc. Of course, I couldn't take it literally. But even today, I don't know what to make of all of this. Did this actually mean something? Afterwards, a guy said that I could probably date her if I wanted. But I rejected the thought because I had no idea whether to take any of this seriously.
• Since then, we've chatted occasionally, and worked together as part of our schoolwork, but nothing serious has happened. However, our relationship definitely changed after it. She started to grin and smile at me whenever she saw me, and was much more enthusiastic about talking to me, which is how our relationship became 'pleasant' like it mostly is today.
• Right now, we're in the same french class, meaning we see each other for at least 4 hours a week. But she's sat on the opposite side of the room to me, meaning we don't chat much. However, we both play the viola and are occasional members of the orchestra, so we see each other during rehearsals and in the music department at times.
• Why do I fancy her? (You may want emotional puke bags at the ready). She's by far the most gorgeous girl I've ever known, she's really outgoing, fun-loving and mature - and lovably sarcastic sometimes (although, admittedly, I don't know her personality inside-out; and I realise that it's not perfect, like with anyone), she's a very clever girl, she has a great sense of style and she's been nothing but really likeable when we're around each other.

I think that sums up most things. Basically, whenever I start talking to her, my heartbeat pretty much doubles. At the very least, it increases by 50%. She's the only girl who I can be around for that to happen.

So my dilemma is: what the hell do I do? Do I try and get to know her better? Do I tell her some or all of my feelings? Do I ignore the whole thing as a common thing that happens to guys my age and wait until later life for a 'real' relationship? Do I tell anyone else?

Obviously, feel free to ask about anything that I've missed. This is the first time I've properly talked about something like this.

And, finally, I'm REALLY sorry for the ridiculously long post.

Ask her out on a date like everyone else does.

if she said she want to marry you and your not going , go for it , I bet its a yes , it sounds like she likes you and 6 years is a long time and if she not with anyone , then hun there is nothing holding back bt ur fear :)