Well, you're not the only 19 year old on the boards, so you're not alone here
Although most of the Bransforumers are older.
The whole niche thing? Yes and no. It's complicated, which isn't helpful for someone starting out.
If you write non-fiction, then yes, having a niche blog is absolute. If you write books about cooking or food, your blog should be about cooking and food. You're establishing credibility and your blog becomes an extension of your book. That's the easy one.
With fiction, it's harder. A lot of writers seem to start out their blogs writing about writing. It's an easy place to start because you're in the thick of it, but most don't stay writing about writing, and that's good because writer blogs tend to attract only other writers. The good thing is that writers, especially writers who are bloggers, are some of the best people you will EVER meet online. They can make for a great community foundation. But at some point you need to branch out or you won't hit readers, which eventually is something you'd want to do.
Figuring out a niche can make things easier, sure. But only if you already HAVE a niche. Let's say you're writing a historical romance set in the 1840s. Your blog might find its niche in history, research, and romance. You might share research advice and resources, fashion posts about the 1840s, the way women were treated, the way they acted in public, men, children, schools. You could spend your whole blog talking about interesting facts, art, and whatever from that time period. You'll attract other history buffs or other writers and readers of historical romance. You probably won't attract anyone else, but maybe that's ok. Because these are probably the sort of people who would read your book. Marketing your blog is easier too because you can go to forums, websites, and other blogs with the same niche, post comments and get involved in conversations.
But having a niche can make blogging very difficult too because eventually you might feel like you are running out topics. There are only so many corset posts you can make and mini lessons on decorum. Being too narrow can give your blog a short life, unless you are a wealth of information and you love your topic dearly. You may never have a very big following, though they may be very loyal and comment often. You will likely only draw people who are already interested in your topic, and maybe not very many people who have never heard of your topic or never knew they had an interest in it.
I personally started out with a pretty narrow focus on storytelling - very specifically books. I eventually expanded to include topics on all sorts of storytelling, but I narrowed my focus in that I write YA speculative fiction so most of my blog topics are sort of geared that way. It is broad enough that I can talk about steampunk one day and zombies the next and most of my readers love speculative fiction in all its forms, so it works out ok. I can also include talking about blogging, videos, music, and art because these too are forms of storytelling. I've got a good background in writing, blogging, and art, so it makes it easy to find lots of post topics. While most of my readers are writers too, they show up more for the speculative fiction talk than they do for advice on writing, which I don't have a lot of.
Kiersten White is a great blogger who doesn't really have a niche, although before she was agented and published she blogged mostly about being a writer on a journey to publication. Now she blogs about whatever, and her readership doesn't mind. The thing that makes her special is her voice. You feel like she's hanging out with you in your living room, not some stranger halfway across the country. She brings something special to her online presence. Same goes for author Maureen Johnson.
Beth Revis, Tahereh Mafi, and Alex J. Cavanaugh are three other fantastic blogger authors who don't really have a niche exactly, but they have something else that is totally worth reading. It's hard to pin down exactly.
If you think you might like blogging, that's awesome! Anyone can do it, but not everyone will want to KEEP doing it. It might not seem very time consuming at first, but eventually it is very time consuming and for some it is worth it and for others, it's not. The way my life is scheduled makes it very easy for me to shoulder the time commitment of blogging. I have a lot of pockets of extra time in my day that are excellent for reading blogs, commenting, and writing blog posts, but aren't very good for working on my manuscript or reading or doing other things. So I use these pockets of time to do blog work and then I don't take away from writing time. Not everyone is so lucky.
Blogs do not have to be a 5 day a week job either though. Sometimes it might feel like it should, but one day a week, or twice a week, is fine too. Consistency is better than quantity.
A final option if you want to think about blogging or trying it out are group blogs like the one I am involved with (with other Bransforumers) over at Wicked & Tricksy http://www.wickedtricksy.com
We have guest bloggers every friday. Margo handles the arrangements but you could always come try your hand at writing some guest blogs for us and see what you think.