Let's start with a basic thing you should think through before you put your pen on the paper, or open your new Word document.
First or Third person?
Most people who start to write begin with a first person point of view. It's natural - we're used to observing events this way, through our own eyes, and telling a story from the perspective of "I" is fairly easy.
A lot of people will tell you it doesn't matter which you pick, and that which to choose depends on how you want to convey the story, blah, blah, blah, blah. Honestly; there is fashion in story telling, just as in clothing, purses, furniture, or cars. If you don't believe me, pick up a bestselling book from the 1960's and compare to one from 2012. At the moment, third person is more common and more fashionable than first person, and some publishers won't even consider a manuscript written in first person. You, as a writer, need to be aware of this before you start, because once you've written 80,000 words from one perspective, going back to change to the other is a pain in the behind.
This said, pick one, and do not switch in the middle of the story. This sounds self evident, but it's not. If you start writing in first person, it will soon be tempting to toss in something that someone besides the main character sees or does. If the main character doesn't know about it, you can't write about it.
Let me try to explain this a little better. Let's assume the main character is named Bonnie. Bonnie starts the story off with, "I sat on the bus," and elaborates on her adventure there for a couple of chapters. Meanwhile, Filip is doing something else. Filip is doing something very interesting. Some authors will try to toss in a couple of chapters where Filip tells the story, "I went into this old mine and found a box with old clothes and a map." If you're writing in first person you can't do that; you can only have one story teller. Few blunders an author can make throws the reader out of the story in the same way as changing the person behind the "I."
Some writers try to put in a portion in third person in the middle of a first person story. This is generally also a no-no. You might be able to get away with a brief portion if it's still told by the main character. In my example, it would be along the lines of Bonnie saying, "Filip later told me he went to the old mine and found a box. While he was there..." If the writer can find a way to explain to the readers how Bonnie knows what happens even though she wasn't present, Bonnie can tell them something about Filip. Filip can't tell the readers diddly.
If you're writing from third person, all this is much easier. Start a new scene or a new chapter and write about Filip. When you're done with Filip, start a new scene or a new chapter and go back to Bonnie. No problem.