Please note, that this is just an example of a classic Mary-Sue. I am not saying that if your OC has these traits, they are Mary-Sues. This is just a small example to keep in mind while creating an OC. This is not an actual character. Apologies if I used a name the same as your OCs, please just ignore that. To help you out if you're a new writer or don't understand the way Mary Sue works, I've written some additional points below which will help you out.
Kirei Aiko (きれい 葵子 meaning "beautiful lovechild").
Kirei Aiko is a child of Uchiha Minoru and Hyuga Haruka. Her parents moved out of Konoha when her mother was pregnant, setteling in the country, hoping to protect their daughter. When Aiko was 5, a group of shinobi came and attacked the house, wanting to kill Aiko because of 10 Tails she has been born with. Her parents were killed, and she was the only one left alive, killing the attackers. She fled to Konoha where she was taken into the local orphanage where she met Naruto and Sasuke whom she made friends with immediately. She was hated as a child and everyone was quite scared of her, as she often killed the children who were bullying her. Several assassination attempts have been made, but they all failed because she kept on killing the ANBU who came for her. With time, Orochimaru heard of her skills and began making plans of kidnapping her, equally likewise to the Akatsuki, who were also after her. Upon hearing the news, the people of Konoha promised to defend her at all costs.
Aiko is often described to be quite beautiful, which is also indicated by her name and surname. Her name was given to her by Kakashi when she refused to join neither Hyuga or Uchiha clan after they were fighting for her. She has two different eyes, one containing Sharingan, while the other has Byakugan. Aiko has waist-long black hair with purple highlights in it and porcelain pale skin. She is usually seen wearing a black knee-lenght kimono, tied with a red obi and black ankle-length ninja shoes. Many girls tend to get jealous over her incredibly petite figure and giant boobs and hips, while she doesn't care what she eats. Ino often stalks her to see if she really doen't excercise or starve herself in order to maintain her looks.
Yes, yes... The original gothic lollita beauty that everyone loves and adores. Two different eyes, purple streaks, pale skin, flawless figure. HOW BORING CAN SHE GET?! Do not forget that she'd most likely also recieve the Hyuga Mark. Yes, your beautiful beauty-chan will be walking around with a green mark on her forehead. I don't think Hyuga clan would just let her walk around with Byakugan unprotected like that...But with a Sue everything is possible, right? Make her fat, or make her have a weird nose. Or something! Skin problems <3 Hair fail! Or maybe her brain won't be reflecting her appearance? Or the other way around? Come on, be creative! Maybe you know someone in real that you can inspire your OC's appearance on? Think about it :) Even everyday people on the bus :p
Aiko is able to use Sharingan and Byakugan on extremely high levels. She graduated the academy at the age of 4 and became a Chunin at the age of 5. By the age of 8 she was asked to become Hokage. Most of her skills are unknown to many, due to her being trained in private and individually by Kakashi and Gai. Gai mentioned at some point when she was a genin that she is above his level at Taijustu, and Kakashi's with the Sharingan. She is extremely fast, which is why it is easy for her to avoid her enemy from reading her moves. She can copy bloodline limits. Aiko has unlimited chakra because of the Ten Tails inside of her. She can control the Ten Tails without a problem, and the demon is always protecting her. She has all the five chakra elements.
Oh hell yeah... Just make her the Hokage of the Naruto universe, since you just made her more powerfull than all the Kages put together. Not to mention that at the moment Mei (the Mizukage) is the only known shinobi with two Kekkei Genkai's (and she's definitely not flawless, that's why we all love her :D)... If you are going to use the idea, do make it a good one. Maybe she can't do melee attacks? Maybe she's can only do range? Maybe she's defensive style only, attacking when it's least risky? Maybe she sucks at defence, but because she has something that lets her regenerate if she buys enough time? There are millions of possibilities! :o