This is not something "a layman" can do. If you want to replace that gear using 3D printing, you'll either need to go down the rabbit hole and learn a bunch of new skills, or hire someone to do it for you.
You need to have a 3D model of the part you want to replace. While 3D scanning is in some cases possible, it won't give you the resolution required to reproduce the gear with sufficient accuracy. This means your only option is to manually model the gear in CAD software. You'll need to take a bunch of measurements (outer and pitch diameter, thickness, mounting hole diameter) and figure out the tooth profile, and then build a model according to those measurements. A caliper is very helpful when doing this.
A neat trick that might help with a flat part like a gear is to scan it on a flatbed scanner, and then use the resulting photo to trace your CAD sketch over (but you'd still need to cross-reference it with measurements).
Unfortunately, most CAD software is quite complicated and "How do I model a gear" is not a question that can be answered concisely. @fred_dot_u offers a number of suggestions for packages that might work, and even offer modules specific to generating gears. However, if you are not interested in learning CAD for its own sake and really get into 3D printing, learning it just for the sake of replacing a gear in a DVD player is a very inefficient use of your time.