Leo Babauta from zenhabits is a good source to go to.
Decluttering was a personal struggle for me, that I think to have handled now. Here my current model for how to de:clutter. Note that I mix actual experience with theory, also some might not be universally applicable. Also I don't know which points to elaborate on and which are obvious.
Preface:
Order is a process, not an end state! Much progress is achieved early on. Like optical decluttering the visible areas, when everything is nicely boxed up. (80:20 principle)
Tools:
I use stackable plastic containers like these. The important factors are the volume that allows to store all kinds of things, transportability by hand, and the possibility to stack them onto them self.
Lots of garbage bags. (Get some of the big ones, some the smaller once9
Tape that can be written on + marker pens.
It might be a good time to install more shelf space
Timing
Depending on your schedule you can use like half an hour each day, or some hours once or twice a week to attack it, and make as much progress as possible. Use a kitchen timer. Get family involved if applicable.
Target areas
Choose a room, or an area smaller than a room to attack. Common rooms or your own are best. (I think that parents should not clean up their kids rooms, safe for fire and health hazards.)
If you are into planning, make a list of all areas and their order, so you can cross them off. Find out which parts of the process give you pleasure and optimize accordingly. (Some people find it helpful to know exactly which steps to take, some get anxious from it. It helps to know which one you are.)
Maybe clean floor space first. And tables.
Depending on level of entropy you can go in one swipe, or do multiple rounds.
Methodology
Put everything that is obviously to throw away in a garbage bag. Put everything else into the boxes. You can do a lazy general sort here right away, but its not necessary. If you do label the boxes with the tape + pen. Take boxes out of the area. Clean the area. Think how you generally want to to lay it out. Put stuff back into the area. Leave everything else in its respective box, till you get into the area where it belongs.
In the end you should have some thematic sections. All office supplies in one place, all electronics. All tools etc. Make your own categories!
In general I find I helpful to know where an item belongs. It should be clear without much thought.
Appendix:
The way to declutter differs widely along what kind of stuff you actually have lying around. Some general pointers:
Lots of good advice here!
I'd also add: don't get bogged down in details.
Many, many are the times I've set aside time to properly clear up and found I've spent an hour sorting through one stack of papers...
I'd suggest: start with the big things first. You can sort which papers to keep and which to throw after you've picked up the bigger things and put them in boxes out of the way.
There's a huge amount of relief in cleaning up even just the easy 80% of the clutter, so tackle the low-hanging fruit first and leave the details for the second pass.
I am beginning to suspect that it is surprisingly common for intelligent, competent adults to somehow make it through the world for a few decades while missing some ordinary skill, like mailing a physical letter, folding a fitted sheet, depositing a check, or reading a bus schedule. Since these tasks are often presented atomically - or, worse, embedded implicitly into other instructions - and it is often possible to get around the need for them, this ignorance is not self-correcting. One can Google "how to deposit a check" and similar phrases, but the sorts of instructions that crop up are often misleading, rely on entangled and potentially similarly-deficient knowledge to be understandable, or are not so much instructions as they are tips and tricks and warnings for people who already know the basic procedure. Asking other people is more effective because they can respond to requests for clarification (and physically pointing at stuff is useful too), but embarrassing, since lacking these skills as an adult is stigmatized. (They are rarely even considered skills by people who have had them for a while.)
This seems like a bad situation. And - if I am correct and gaps like these are common - then it is something of a collective action problem to handle gap-filling without undue social drama. Supposedly, we're good at collective action problems, us rationalists, right? So I propose a thread for the purpose here, with the stipulation that all replies to gap announcements are to be constructive attempts at conveying the relevant procedural knowledge. No asking "how did you manage to be X years old without knowing that?" - if the gap-haver wishes to volunteer the information, that is fine, but asking is to be considered poor form.
(And yes, I have one. It's this: how in the world do people go about the supposedly atomic action of investing in the stock market? Here I am, sitting at my computer, and suppose I want a share of Apple - there isn't a button that says "Buy Our Stock" on their website. There goes my one idea. Where do I go and what do I do there?)