I know it's hard to start – it can seem overwhelming. On the website, I advise getting some containers (boxes, bags) and paper & markers for signs.
START the organizing process by being prepared. Then take those containers (usually marked KEEP, DONATE, TOSS) and start with one area (a small self-contained area) like a closet, drawer, cardboard box full of stuff.
Then devote some time – 1/2 an hour or 1/2 a day – to complete that one task. Take the Donate stuff to a local drop-off (ideas on the website) or have some organization pick it up (ideas also on the website).
The "Toss" stuff is recycled or trashed (but, please, do it responsibly). The "Keep" stuff is cleaned & tidied and put away neatly (you may want to get some shelving or bins to keep things tidy). That's it. It's a start.