Real people have quirks. I recently heard a story about a girl that was a nail artist with inch long fake nails and sprayed on designs that was also totally into the American flag and Cheeze Wiz. They say you can't make this stuff up - but you can. Writing believable characters might require you to start grabbing all these crazy anecdotes you've heard, filing them away, to bring out later and mix and match in your writing. One of my latest collectibles is about a woman that picked the lock when her guest was taking a shower because she thought someone left the water running.
I recently visited the house of someone that decorated their house with a combination of naive art and antiques, while feeding all of the neighborhood stray cats. They spent a fortune on cat food for animals they didn't own and couldn't pet. Or the wonderfully casual comment from the rich guy who has a huge house with multiple bedrooms, swimming pool, and a crew of migrant labor to clean his grounds and when you describe your 650 square feet of living space says "oh that's plenty big enough for two, what more do you need?"
If you want to be a writer you have to start to develop a strong streak of curiosity, a certain amount of objectivity (i.e. be amused by the comment by the rich guy and file it for later instead of popping him in the face), and a good memory - or a good filing system. Remember to avoid clichés. One person might like to bathe every day and moisturize their skin twice a day while another person might forego bathing for days yet they both are obsessed about beauty and aging. Pick the set of character traits that serves your character best, and preferably the one that is less common if it works. The important thing to remember, regardless of the well-worn adage that "fact is more unbelievable than fiction" is that if you can think of it it's probably true somewhere so just write it with conviction and you'll bring your readers with you.
Speaking of aging: older characters tend not to be as popular with collaborative fiction writers. Very often writers go for the young and physically perfect. It's good to remember that young people simply don't have as much life experience or cumulative time to pick up wonderful idiosyncrasies as older characters (though my example of the nail artist was a young woman). Older characters can provide a level of depth to your writing that might be lacking from your typical young and nubile. Adding just ten years to a character's age can result in greater opportunities for peeling back the layers of your character's personality to keep the reader engaged.
A character doesn't have to be likeable but they do have to be fascinating to keep a reader's interest.