I'm going on a job interview next week and it's really important. I already started preparing it and I came across this possible question:" what are your strengths and weaknesses". Could somebody give me some ideas of good answers?

No one can answer that question but yourself. What are you good at, and what are not good at.

A high school counselor gave me this advice many moons ago concerning that question:

Step 1: Sit down in a quiet area and clear your mind with a piece of paper and a writing implement close at hand.
Step 2: Think of the things you love to do. Write those down.
Step 3: Clear your mind again.
Step 4: Think of the things you hate to do. Write those down.
Step 5: Look at your list. Why do you love or hate the things on your list?

As you concentrate on that last step, certain truths reveal themselves to you.

Maybe you love gardening, but are you good at it? If so, why are you good at it? Is it because you are very persistent in picking out every single weed every week so that the flowers don't have to compete for food and grow big and beautiful?

If so, your strengths learned from gardening that you can reveal to your employer are:
You are persistent
You are thorough
You will stay with a task until it is completed

On the flip side (and still using gardening just to give two sides on the same coin), do you hate gardening because it's boring to have to keep doing the same thing week in and week out? Do you get angry with the flowers because they just don't bloom when you want them to?

If so, then the weaknesses you could reveal to the employer are:
You don't like routine.
You can tend to be impatient.

Your strengths and weaknesses aren't necessarily all just plusses and minuses, though, as they can sometimes help an employer to give you a position within the company that suits you well. The person who can't stand routine shouldn't be stuck into a position that involves nothing but filing day after day, but the person who is persistent and can stick to the job until it's done would be perfect for that same position.

Good luck on the interview, and make sure to take a few deep, cleansing breaths to steady your nerves on the way in. You're expected to be a little nervous, but if you are calm, cool and in charge despite the nerves, it's always a great first impression to make!

First things first,
Take a piece of paper and judge yourself critically, don't be too harsh or too lenient on yourself.. try to look at yourself objectively... If you need ask one of your close friends for help... Consider your human skills, technical skills, the works
Then prepare an answer to this question... Weed out weaknesses or even strengths that you think will not be appealing to your prospective employer...
Try to come off as natural when answering it (being a mindless parrot is definitely not a strong point )
I came across this web page which pretty much answers job and interview questions.. It might help with this question and others you might have too... Identify your strengths and weaknesses.

Best piece of advice I can give you is do not say your weakness is being a perfectionist. I hate that nonsense, its never true it just sounds good.

Reframe your workplace weaknesses into strengths. E.g. "I don't participate at meetings" = "I'm a good listener and think things through before I speak."

Beware of the question, "Name two weak points of your previous boss."

I have to spread some rep wondergirl...

Good points, you have to highlight a weakness in a way that its not bad or a weakness that's not really detrimental to what you do. Like I work in HR and my weakness? I care too much about my employees and sometimes it can make me nuts trying to do right by them. Yes its good and bad thing. Those are the sorts of things you need to look for in weaknesses.

You should definitely not say things like "I don't set time limits for myself" or "I show up drunk for work" (kidding!) but you get the point.

And never, ever bad mouth a former employer.