If you're dealing with a huge mess, it can be hard to even know where to begin. Minimalist websites will tell you that your problem is too much stuff, and the only way to clean a messy room for good is to get rid of most of it and embrace a simpler lifestyle, but that can backfire on you: if you learn to associate cleaning with getting rid of things you love, it may be even more difficult to get started.
It's possible to beat the mess without getting rid of everything, and you don't even require much in the way of fancy cleaning tools to do it. At the bare minimum, you'll need some trash bags, a laundry hamper, and the biggest cardboard box you can get your hands on. There are some other things that are very helpful, but not necessary, like all-purpose cleaner, a duster (microfiber works best), and either a vacuum cleaner or a broom to clean your floor.
Now, start from the top of the room, with the highest shelf, cabinet, or drawer you see, and work your way down. For every item you see, ask yourself if it's where you want it to be. If the answer is yes, leave it where it is. If the answer is no, then take it down. Trash goes in the trash bag. Dirty clothes go in the hamper. Dirty dishes go in the kitchen sink. Anything you want to keep goes straight to the cardboard box. It doesn't matter if the place you want to put it is just a few feet away; put it in the box anyway. Worrying about where something goes will ruin your momentum, so keep your options limited: box, trash, hamper, or sink.
If the surface you're working on is dusty, then run the duster over it. If it's sticky, spray some cleaner on it, let it sit for a minute, and then wipe it away. By working from the top down, you make sure that all your stuff has a clean place to go when you're ready to put it away.
Be sure to take breaks and reward yourself for your progress. If you associate cleaning with short, manageable chunks and small rewards instead of hours of exhausting work, you're more likely to clean more in the future, so try not to go for more than an hour without a break.
When you reach the floor, keep using the same technique, but focus on just one section of floor at a time. The whole floor may seem big and overwhelming, but it's not hard at all to just pick up the mess by the door, or in the corner. Once you finish a section, you may want to sweep or vacuum before your break, so you'll have a nice clean bit of floor to feel good about.
Once the floor's clear, take out the trash, and take a break. Give yourself some time to be proud of your progress, then go wash the things that need washing (clothes and/or dishes) and put them away. Take another break. Now turn your attention to the cardboard box. Pick up the first item you see, and put it away. If you can't decide where something should go, just set it aside for now, and put something else away. Go through, item by item, until everything's either been put away or set aside.
If you have any items you've set aside, look at them now. Could you move the things you've to make room for these things, or do you need more storage? Are there things you don't use taking up space where these things could go? You can keep the leftover items in the box temporarily, but make finding a permanent place for them a priority, even if that place is some stacked milk crates in the corner, or a shoebox under your bed. Once the cardboard box is empty, you can collapse it, and either recycle it or stash it in the back of your closet for future use.
Once a week, go back and do another top-down review of the room to check that everything is where it should be, and your surfaces are clean. This keeps the mess from building up, and if you keep it up, it won't take you more than half an hour to get everything straightened up again.
It doesn't take a miracle to clean a messy room: just patience, and a strategy for taking it piece by piece. Once you have that, there's no mess too big for you to tackle.