That smell indicates sulfur in your water supply. There's nothing to worry about. You don't need to do anything, but if it bothers you, use one of those freshening blocks that you hang in your tank, or just put an air freshener on or next to the toilet.
It sounds like you have hydrogen sulfide producing bacteria in your water supply. I am assuming your water supply is on a well rather than public water. If this is the case, you need to bleach your well and all taps to get rid of the bacteria.
My first suggestion is to contact a company to have your septic tank flushed. The second suggestion is to have a plumber come out and clear the pipes with a mechanical auger to clear the drainage lines.
Usually that type of smell is created because of the failure of the wax ring under the toilet. It's actually a pretty easy procedure tp pull the toilet from where it is now sitting, remove all the gunk from the old seal, replace it with a new wax seal, and reseat the toilet.
You should call a plumber to take a look at it. The smell might be methane. You might have a leak in your pipe or in the seal at the bottom of the toilet.
If you have a septic tank you may need to get a product such as Rid-X to add beneficial bacteria to help digest waste that may be backing up in the system.