Hi Meg!
First of all, as you already noticed, not everybody will find you attractive - and that's normal and it's the case for everyone. Some people will find you attractive, and some won't. But you can look at it like this: if out of 100 people, 90 people find you attractive, then you can consider yourself a beautiful girl. If it's 50 out of 100, then you're at least very cute. But if it's less than 10 out of 100... Well you get my point, I hope

Now the thing is this: it's not only the way you look, it's also the way you talk, how you dress and how you carry yourself. All of these factors contribute to your being attractive to the opposite sex (positively or negatively, depending on what you're doing). This is true for both men and women, by the way.
The good news is that you can improve on all of these aspects. Improving your looks: start working out for example, start playing sports. Improving the way you dress might take a little investment on your part. As for how you talk and carry yourself, I'd suggest getting some guy friends - and start practicing, talk to them, see what works and what doesn't (Alternately, you can watch how another girl does this, one you might consider "successful" with the opposite sex). Start experimenting.
Hope this helps!