Okay, to start off...

When you want to create a new character, you need to think about who they are going to be, what their role is in the story, and a little bit on their back story.

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Are they going to be male or female? If so, what will their name be?

Depending on these, you're going to need to determine their heritage, or personality, to find a name. For instance, if they are going to be related to Leo, then they are most likely Hispanic and would need some sort of trace of Latino in their name. Martin, Gabriel, and Rio are examples.

Or, if they are a spunky, upbeat person, then don't give them a name that's depressing, like Imogen. No. Just don't.

Here are two GREAT resources for names:

Boys: http://www.randomnames.com/allnames.asp?g=m
Girls: http://www.randomnames.com/allnames.asp?g=f

Determine personality traits for a strong, easily identifiable character.

So, you really want to go into detail if you want a great person. So, here are two great, really thorough character outlines that I use when I make a new person. (Scroll down a little to find the whole thing.)

http://www.epiguide.com/ep101/writing/charchart.html
http://kittyfelone.deviantart.com/art/Character-Profile-Outline-45504357

One that is a LITTLE shorter:

http://yeahwriters.tumblr.com/post/24774343500

If you're lazy, then try this: make at least THREE traits that your new character will possess, and make sure to expand on them, (AKA character development) as time goes on.

Will the new character be a protagonist?(Main character)

If so, then the character outlines above I will totally recommend using because you really want your main character(s) to be detailed so readers will become intrigued and want to learn more about them.

If not, then follow the "if you're lazy" bit. You want your secondary characters to be interesting but not that insanely detailed.

What is their role in the story?Watch out for accidentally changing character and personality traits...

Okay, so this is important. If your new character is a girlie girl, then don't make her a fighter in the next chapter. If your new character is a tough guy, don't make him weak in the next paragraph...be consistent, check to make sure that your boy's eyes aren't blue in one section and green in the next.

When you aren't used to someone, make sure to refer to an outline or some sort of note to make sure that the sarcastic one doesn't magically change to the silly one.

Notes

So, I hoped this helped you guys out so far! Please ask me any other questions if you have them.