The things you mention can be quite easy to do, but, when I'm beginning to learn something, I frankly become rather annoyed by people who've been at it for years telling me how simple the work is. Some of the equipment you buy might come with installation instructions; your success, then, could come down largely to how well-written/-illustrated these are (I, for instance, hate many of Ikea's furniture-assembly diagrams). Here, however, is a quick run-through of the specific jobs you've mentioned:

Saddle: it's held onto the seatpost (the metal rod directly attached to the bottom of what you sit on) by a clamp assembly (part of the seatpost) that bites onto a pair of rails (permanently fixed to the underside of the saddle). When you buy a new saddle, it won't come with a new seatpost; you'll get just the thing you sit on, with its own pair of rails, underneath. What you'll do is loosen the clamp assembly (using a screwdriver), slide out the the old saddle from the seatpost, slide the new saddle onto the post, and tighten the clamp around the new saddle's rails.

Lights: if battery-operated, they often come with some little assembly that can be wrapped around the handlebar of the bike (for a headlight; a taillight typically is placed along the length of the seatpost, or on the rear rack), then tightened with a screwdriver. The complexity of installing bicycle lighting tends to manifest when you're using lights that are run by a generator powered by the bicycle; matters are further complicated if you choose to run the wiring (from the light to the generator) through the inside of the bike's frame (which is hollow), rather than over its outside.

Cyclocomputer: I don't use one, but I have seen a few, and the difficulty of installation tends to be greater than that for a saddle or for battery-run lights, but it still is something you can do yourself (i.e., you don't tend to need any special tools or a strong amount of knowledge or dexterity).

I will say, most bike-shops, like auto-shops, make a good deal of money from selling labor (fine, the car folk rely on this a lot more). Unlike with an automobile, though, working on many aspects of a bicycle is something most people can handle on their own, if they get over the initial intimidation. Unless you're talking about something like building a wheel — which even professionals need plenty of practice to get good at — you'll probably be able to manage a lot of your bike-stuff, yourself.