This is obviously not a project you can do yourself unless you are familiar with it and have very specialized equipment to do the job. And of course if you are installing a new septic, you need to have the hole dug, septic system bought and delivered, and some way to get it into the hole and get the drain fields properly built. It is near impossible for a DIYer to work with septic systems.
For this reason, a site where you can find a ton of information about this kind of work is Knee Deep Septic. This site can provide you with the details on how septic systems work as well as the costs and complexities involved with getting the system properly installed or pumped out.
One thing you should strongly consider if you have an existing septic system is a green cap installed on top rather than digging the grass up every time you need to get into the thing. The green cap blends nicely into the lawn (assuming you have a lawn on top of it). You can run a lawn mower over it without doing any damage to it and the next time the system needs to be serviced, the technician simply lifts the top off and does the job. No digging or unsightly dirt piles will remain.
These caps typically cost about $125 or so, but the cost is well worth it.
You should have your system serviced every 3 to 5 years. Don’t wait for it to back up before getting this done. If you see water seeping out of the ground and pooling in the yard where the drain field is, or notice your commodes are starting to make a gurgling sound when they are flushed, it is past time to have the system pumped. Check out the website above for much more detail on the subject.