I'm a big fan of neutral colours and I too prefer to have things on the walls that are meaningful to me.
Start by having a big clear out. You might be able to sell stuff on marketplace or if it's unusable, take it to the tip.
Pick a colour scheme. Do you like neutrals or bold, bright colours? I tend to go for whites/greys/creams and chrome and I try to only ever have one pattern. So if the curtains or carpet are patterned, I keep everything else unpatterned.
For pictures...I'm by no means a photographer but I have a load of family photos that I turned to black and white (with a photo app) and I decorated a whole wall with them in matching white frames. Whole thing cost me about £50. You can buy white frames in bulk online. I also have a friend who's an artist and as a housewarming gift, she drew me a picture of the front of my house...which is now in a frame in the hallway. I also have a couple of quotes that I like in frames in the spare bedroom. I designed them in Canva and printed them using vistaprint.
Kitchens can be totally renovated with just some cupboard paint. If you have tiles that you hate, you can buy tile stickers to cover them up.
I bought some really good quality curtains from B&M yesterday...floor length heavy blackouts, reduced to £50. But also Dunelm often have good deals in-store (that aren't advertised on their website).
I tend to spend most cash on the stuff I'm keeping longest, so sofa or other furniture but always accessorise from places like Dunelm/B&M etc. Also, recent discovery is that bigger B&Q stores sometimes also sell cushions and stuff. I've had some great bargains from there.
I think the biggie is having a big clearout first of all and then you'll get a proper view of what you need.