There are a few things to consider and it's important to get these parameters before we get started
1. Performance Requirments/Specifications
Asking precisely what you want this PC to be able to do may seem like an obvious question, but it's important to think this through. For example, if you want to run new games in 4K for the foreseable future, you'll need a more powerful and expensive machine that one that with lesser ambitions, like 1080p or 2K gaming. Do you want a multi-monitor set up? It's also important to consider if you want the computer to function as a multimedia system or as a utility for video editing, audio production, streaming, recording and so on. Have a good thing about what exactly you want.
2. Whare are you Planning to put it?
I know it sounds stupid, but specifications and parts lists can vary greatly depending on where you plan to place your PC. for example, if you plan to use it as a living room PC you might not need monitors, but will likely need Bluetooth and WiFi compatibility, whilst a more traditional desktop might not.
3. Budget
And don't just think about the actual PC itself either. You may need monitors, periferals, accessories, a desk, chair and so on. These cost money too and can be a significant chunk of your budget, but are very easy to forget. Obviously you want a PC that's suitable for your specific requirements and that's as future proof and capable as possible within your budget.
4. Aesthetics
Do you have any aesthetic requirements? I know this sounds trivial, but one of the advantages of building a PC is that you can have components match to a colour scheme to suit one's personal tastes and/or match the room one plans to use it in. It's unlikely to alter the cost and can make a PC really feel like it's yours.
Have a think about this and get back to this thread when you're ready.