If you look at the big picture, you will get overwhelmed; so you must break it down into small spaces. Or, just begin in one room. Just dedicating an hour or two a day will net you with great results and will keep your momentum going. If you think you can clean a really messy room in one day, you are setting yourself up for failure and you will give up.
So, to begin: Get three boxes or bins and label them "PUT AWAY", "THROW AWAY" and "GIVE AWAY". Beginning on what is on the floor in one corner, pick something up and put it in the appropriate labeled box. Keep going until a box is full. When the box is full, do what the box says. For the "Throw Away" box, dump it in a bin outside. For the "Give Away" box, put it either in the garage or load your car for the donation boxes that are typically in local parking lots. For the "Put Away" box, put things where they belong. Then, get back into the room and carry on. Once the floor is clear, then do the same thing with whatever is on the furniture. You will be amazed with how much you can get done in one hour.
Vacuum the floor, dust the furniture and then move onto another room. You can back track later to go through every drawer and closet by using the same three box method.
Set reasonable goals and understand that it took a long time for your house to get messy, so it is going to take time to get it organized. The trick is to have a place for everything and keep everything in it's place. To maintain a clean room, every single time you leave the room, look over your shoulder and make sure everything is where it belongs. This is just a matter of changing your habits and really takes very little time to keep a clutter free house on a daily basis. It is when you stop the little bit of daily maintenance is when things begin to pile up again and you have hours more time ahead of you to get it cleaned up and decluttered again.
Get rid of things you don't need. Sentimental things are a burden sometimes. We all tend to attach a memory or a person to our things that we really can live without. These things weigh us down. Sure, it is nice to keep some things, but you need to prioritize and just keep things that are near and dear to your heart.
I recently organized every drawer in my home. I cannot believe the crap I threw out and gave away. It was not always easy, but now everything I need I have and it is all at my fingertips because everything is organized. To be organized makes life so much easier on a day to day basis. At one time I could never find my scotch tape, so I bought more and more and more. When I organized, I had five rolls of tape. Now, that is a colossal waste of money and space. I ended up getting rid of almost a shoebox full of pens and pencils and magic markers. We do not need all that stuff taking up valuable space in our homes.