With regards to scholarships, it's important that you understand the basic structure of the system before you even start applying. After all, while the scholarship system is meant to help others, it doesn't always manage to cover all those in need. You have to work within the system to show them that you need help, and its hard to do that if you don't even know how the system works yourself.
The goal of a scholarship is to help someone who has worked hard, needs help, and even the playing field for those who are coming from historically discriminated against backgrounds. So scholarships are divided into three categories based along those lines.
#1: Merit Generally, this means academic merit, although athletic merit also falls under this. This is the section that seeks to reward those who have worked really hard and have potential with a scholarship.
#2: Financial Need If you simply don't have the resources to go to college without burying yourself in a mountain of debt, this branch is meant to help you.
#3: Diversity These are the scholarships meant to help those who have been discriminated against, and encourage diversity at colleges. This includes scholarships for those of certain racial/cultural backgrounds, genders, sexual orientations, disabilities, and the like.
Sounds straightforward enough, right? If you don't have significant financial need, you can get a merit scholarship, and even if you're not so much a scholar, you can still get aid based on your financial need. Not so fast. You see, these three categories overlap with each other a lot. Finding a scholarship based on ability, financial need, or diversity alone is like finding a needle in a haystack. There is almost always a GPA you'll have to meet, even if it's a mere 2.5. Increasingly, academic/athletic merit and diversity scholarships come with the requirement that you have some degree of proven financial need. All of these can prove to be problems for applicants; some can be solved, but others are far more difficult.
The one element of scholarship earning that I have yet to be able to solve is the need for a GPA requirement. It's unsurprising that there really aren't scholarships with no grade benchmarks; after all, scholarships want to help those who have the best chance of succeeding in college, which are generally (while not always) those with the higher GPAs. So this is the critical element of getting a scholarship: keeping a decent, if not high, grade point average. I'm talking at least a bare minimum 3.0 GPA, although I would aim for a 3.5 if you can manage it. If yours isn't that good, yes, you can try and explain it in your essay, especially if there's good reason. If you were hospitalized for a year, had a death in the family, or experienced something equally dramatic that would have impacted your grades, these are all good explanations. In those cases, you would simply explain what had happened, and then, rather than getting stuck talking about the problem, focus on your previous accolades in school and your goals going into the future. Beyond something extreme like those, though, an explanation probably isn't going to cut it. Your best bet is to keep your GPA up in the first place.
The thing about scholarships that I cannot emphasize enough is the necessity of applying. You will not get a scholarship if you don't submit an application to it. Period. So submit as many applications as you think you can write while maintaining a high quality. Most of all, believe in yourself. If you don't earn one scholarship, it's not the end of the world. You just got some feedback, which you can use to help you when you apply to the next one. It can be disheartening, but the effort truly is well worth it if you earn even one scholarship.
Finally, scholarships aren't only something you can apply for your senior year of high school. In fact, they'll continue to be available for your entire college career, both for undergraduate and graduate school. So keep applying - your efforts will pay off.