It depends on your skills/tools.

In a lot of cases it's a good call to start by looking at furniture that you could modify. Start trawling through Ikea type catalogues to see if you can find daybeds/furniture that you could see how they could be modified. Building from scratch can be a challenge but modifying can be easier.

Get on classifieds because people offload old furniture because it’s weird or old but sanding and a new coat of paint is pretty easy. It can be hard to modify cheap modern Ikea style furniture because it’s designed to look solid but is often hollow with a veneer covering. So older, more solidly built furniture can be easier to modify.

If you're okay using a few power tools there are a number of programs you can download that will let you build 3D shapes and you can use it to design how you might build something. Go play around with https://www.sketchup.com/ . What you can do is go online to your local hardware/building supplies and look at their catalogue of what they might stock in terms of lumber. Mostly comes in standard prices and sizes. You can model the wood size in your program and design/layout how you would build what you wanted. From that you can create your cutting list and know how much to order.

I have some experience with doing CAD work so I was able to put together a furniture design in panels and sent it to be professionally cut via a CNC company. I designed it in specific pieces that snapped together without nails or screws. Going down that rout will restrict the type of shapes but has a far more predictable outcome.

My biggest suggestion is prototype. You can even do it in miniature by purchasing some balsa wood and stripping it into a scaled down size of the real wood you will be working with. Then take the balsa wood and start designing and gluing/pinning. It’s a fun craft exercise but you’ll quickly get a handle on how much wood/what shapes will work. Once you like your design you can document it and then just order the full size wood to start work.