Echoing the "pick one thing and start from there" advice. And think about what type of house you have – don't try to do jewel-toned Victorian grandeur in a 1960s ex-council block, for example. If the house has certain features, follow them.
For example, the sitting room of my flat has a HUGE square Edwardian bay window, a fireplace & a shell-effect Artex ceiling. It's also a tiny room. So I made the window a feature by choosing patterned curtains, which form a big block of print when closed, almost like a feature wall in themselves. Then everything else – ceiling, walls, fireplace, door, woodwork – I painted shell-pink. One colour on everything = tricks the eye into making it look bigger. But I'd have done something completely different in a different space.
Pinterest, Pinterest, Pinterest, then try to figure out what appealed to you about each pin and what the overall trend is – do you like Scandi neutrals, shabby chic, mid-century, etc; or is it that every pin contains a great rug or statement tiles?
You can't go wrong with unifying your house by picking one flooring throughout (so no door thresholds collecting dirt), and one paint colour throughout, and using that as a neutral backdrop.
And remember it's ok to make mistakes! If you get the wrong rug you can return it or eBay it. Walls can be repainted. Lampshades can be changed. Sofas can be reupholstered or, more cheaply, covered in a different throw and cushions. Move art around, rearrange your decorative objects, experiment. It's a work in progress, not a permanent Instagram.