My advice would be to avoid using characters that aren't yours entirely. Try to create your own, if you fail at that step then it's likely anything you write even with borrowed characters won't be anything special either.
The other side of the coin on that is that you'll know instinctively how characters you've created will act or react to situations, dialogue or internal conflicts. When you take someone elses character and start using them, you may end up using them in a way you perceive to be correct but the original author would find completely wrong.
As to the rest... all of that is part and parcel of writing. If you can't do any of that with your current writing skills I'd say shelve it until you can develop some writing skills to begin with.
The start of that of course is to read. Read other people's fanfics and read books most especially. See what works and what doesn't, then emulate that.
Some pro tips.
1. Characters.
Think of who you'll want and what story you'll want them in. Check
Here or
Here for some stock archetypes you can use, just don't forget that archetypes aren't the be all and end all of characters.
2. Plot
You have a story. Great. Now you need a plot.
What?
Yes. A plot. Every story has a plot. It has a beginning, a middle and an end.
Beginning: The Trouble Begins
Middle: The Trouble Escalates
End: The Trouble Has Been Vanquished... or it ends with even more escalation resulting in a cliff hanger.
This is of course a very basic plot. Most writers plot out their story from start to finish subdivided into these three subdivisions. You can take any plot from a book, a film, even a webisode short and still come with a beginning a middle and an end.
Heres an example on how I'd plot out a story I'm making up as I go along.
Beginning
Homeless little girl all alone in street
Sees advertisement for intergalactic space cruise
Little girl stows away in transport craft
Middle
Little girl on spaceship
Is found by crew
She runs
Eventually is captured
End
Crew decides what to do with little girl
Realises she is homeless
They're on the way out of the solar system, turning back is pointless
Crew adopts little girl
The End.
All that needs to happen is to write a story that covers all those points. As you write, new scenes might pop up in your head, the plot may even change completely depending on how you want to spin the story. As long as you have a plan and a plot, you should be on track.
3. Practice
Nobody got good at anything by not practising. Start writing and keep reading. Draft and redraft if you need to, as many times as you want until you're either sick of looking at it or are happy with what you've got.
Post what you've got for public review and say you want constructive criticism.
Now here's the most important part.
When people criticise your work constructively, TAKE WHAT THEY SAY ON BOARD. I've lost count of the number of times I've seen someone blow up because somebody gave them an honest appraisal of what they wrote or simply IGNORED what was said because it wasn't what they wanted to hear. If someones taken the time to give you some concrit, take it on board.
4. Don't fall in love with your work.
Exactly what it says on the tin. You might have a great line, a great scene or even a great chapter... but if everything around it is crap, don't be afraid to throw it out or forget about it.
Your idea may be great, you may have some killer dialogue or a great character in the works. But if your current story just isn't any good according to your concrit, do try to look at your work with a critical eye and don't be afraid to take a knife to the sections that aren't working.