So as for a website or a mobile app project, you need to draft a document before starting to jump within the creation of your digital device. I know you are very excited about trying your code in Arduino/Raspberry Pi, but the point is that if you don't make the effort right now to draft something you will end up by purchasing the wrong components, you may also realize that this project can be done another way and so on and so forth. To avoid this, here are the main questions you need to answer to before starting.
You need to clearly define what are the final expectations of this IOT project. What should it do? Be as precise as possible here. For example, saying that the digital device need to notify you is not as precise as receiving an email.
This part represent the budget that you plan to spend and the list of the components you will need with links to the different components you are ready to purchase. And how many units will you need?
Think here about all the potential challenges you will have to face. Working with IOT is really different than pure digital project. I remember a project of mine that I thought carefully in terms of digital, it was robot, though I did not think about the physical side of it, as a result I ended up with a robot stucked at the middle of the garden of my neighbors because I did not plan that the grass floor is not flat at all so the engines of the robot had to be more powerful than what I planned initially.
Ask yourself about the different technologies you would like to use for the project. Not all technologies are equal, so to say, you may realize after 70% of the project that you cannot progress because the technology has been discontinued, the technology does not support the main component of your project...
Plan and wireframe
This part is about drawing the different wiring to your board and show that you already have a clear idea about how it looks like in terms of connections. You can take a picture at this step if you cannot find a suitable software.
If you plan to use a mobile app with your digital device. Then indicate here what the app will look like.
Think about what should be the final result, is that a box? is that a sphere? and write here how you will design the shape. Will it come from a cereal box? Will it be printed from a 3D printer, will you let it as it is?
Show here the code or code samples that you plan to use to create your digital device.