I will start by saying that I am a naturally organized person. I like things to be labeled, put in nice boxes (whether literally or figuratively), and I don't like having a bunch of unnecessary things around. Being this way helps me keep my house free of clutter. I know that not everyone is like that, but if you would like to learn to be less cluttered, I think these ideas can help.
A lot of people get the idea to become clutter-free all at once. That is a mistake. You will get overwhelmed, not know where to start, and spend a lot of money. That is if you even get started. The key is to start small. Find an area that is manageable like your car, a countertop, a small closet, a drawer, or a cabinet. Once you get one area decluttered, you can move onto other areas more freely. Plus you will take some basic steps and repeat them and it will become easier as you do it.
The first thing you need to do is get an empty trash bag, 2 empty boxes, and an empty laundry basket or bin. The trash bag is for just that, trash. One of the empty boxes is for things you want to donate to Goodwill or some other organization. The other empty box is for items you need to return to someone or think someone else would like (besides Goodwill) or things you can sell. The empty laundry basket or bin is for items that go somewhere else (ie it's in the car but belongs in the house).
Secondly, you want to go through whatever area you have chosen and throw away all the visible trash (mail, papers you don't need anymore, trash) away. Then you want to go item by item and ask:
1) Does it belong here? If the answer is yes, keep it there. If the answer is no, ask the next question.
2) Do I want to keep it? If the answer is no, put it in the Goodwill box or the give to someone else/sell box. If the answer is yes, put it in the laundry basket or bin to be returned to the proper location.
Then, you re-organize the area with the things that still belong there. Leave some empty space so things don't feel cluttered and you have room to add things.
Lastly, you want to return the other things to their respective homes and/or take the stuff and give it away/sell it.
Once you declutter, don't let the clutter come back. Throw out trash. Periodically collect things in a laundry basket or bin and return them to their homes. If you do this once a day, your clutter will be under control.
*Note: A lot of people have things that they really don't need. They struggle with parting with them because someone gave it to them or they bought it and feel a responsibility to keep it. My thought is if you don't need it, use it, or love it, get rid of it. It's not serving you by cluttering up your home, your mind, and your life. Plus there is probably someone who needs/wants what you have and you can bless them or make some cash to get something you really want.
I hope this helps!