Make a plan. Think about how much storage you need, whether you need redundancy and backups, and what server applications you need to run.
Here you also need to consider how much time you want to spend on maintenance. Premade solutions like Synology are set and forget. Using NAS operating systems mean having to do regular updates. Using just Linux is also an option if you want full control and are confortabile with the command line, but it’s more work.
Then step 2 is getting the hardware.
My recommended route, if you want to spend the least amount of money, is to get a 2nd hand PC, preferably not pre-built (dells and hps have proprietary parts like power supplies). A 4 core cpu from the last 10 years should be fine, and 16gb of RAM is more than enough for most applications. SFFs or MiniPCs might be ok for nvme SSDs only, but if you want hard drives, get one that has enough HDD bays. Depending on what you find, you might need to replace a few things, like the power supply or case.
Servers are good and reliable and have nice features, like network management, redundant NICs, redundant power supplies, but are usually pretty noisy.