I agree you start with one smart device at a time. You’ll need to know how it works to be able to run it anyways. It’s all very daunting at first but you eventually get the hang of it.
To start with using Apple HomeKit you first need a Home Hub; an Apple TV Box or HomePod: Set up your HomePod, HomePod mini, Apple TV, or iPad as a home hub - Apple Support iPads can no longer be used once you are on the new Home architecture. You need to have the Home App on the device you’d like to control it with. Old Apple devices might not work. We’re assuming you use Apple devices. A HomePod requires that you have an iPhone to set it up. It can be done with an iPad too but an iPhone is best. You need to be in agreement that you’ll use two factor authentication (some people don’t like this) and have keychain turned on (this stores passwords)
You need a home wifi network. Some devices prefer 2.4 GHZ over 5 G. I use both.
Following this you will need to purchase smart devices that work with Apple Homekit. You will see this logo on the packaging.
I’m sure there are Nerds you can hire but you’re handing over your Apple ID and wifi password to have them do it for you. Every device requires you entering these credentials. For things like thermostats you can hire an electrician to install it. I had my furnace guy do mine while he was here for the annual tune up. For other things like door locks, whoever installs this things are likely to understand smart home set up and can get you started. Bulbs are easier to do. Do your research on what people say about each device you’re considering purchasing. Pros and Cons, does it require Thread and Matter tech to be in the Home Hub. Basically you are going to have to read up on all this stuff!