CAD is Computer Aided Design. CAD is expensive at the start and is usually expensive forever. You can reduce your cash outlay by using free open source software. Here are some ideas.
Commercial software can be cheaper for some projects
Professionals who use software are often paid far more than the cost of software. They focus on saving time, not money. Their favourite commercial software saves them enough time to pay for the software then make a big profit.
Projects involving many professionals are usually run with the software already known by the majority of the professionals because it is cheaper to buy that software than it is to train all those people.
When you work on a specific design, perhaps the drainage system for a housing estate, there may be one commercial product that makes that type of design many times easier.
Free software is not always open source
Software is often offered with no up front purchase price. When it is also open source, there are no ongoing costs. The free proprietary products cost you money down the track. You might find add-on extensions are expensive. You often find you cannot complete a task using the fee version and have to buy an expensive "professional" version. Software is not truly free unless it is open source with a license that lets anyone create their own version.
Open source software is not always free
Open source software can be so complicated, you need professional training. Open source software might require upgrades to your hardware or operating system. For something like video processing, you will need a massive increase in disk capacity.
In most cases the extra costs connected to using open source software will be the same as commercial software. For example, open source video software will require the same disk space increase as commercial video software because the bulk of the space used will be the videos, not the software. Talk with people who are using the software. Ask them about the money they actually spent.
2D or 3D
CAD can produce designs in 2D (2 dimensions) or 3D (3 dimensions). 3D is more complicated and the software more expensive, both in price for commercial software, and the resources needed to process the designs. 2D is all you need to layout a floor plan. 3D is needed when you start to show finished designed with things like decks over a sloping garden.
Start with just what you need, learn, then step up to the next level if you need it.
LibreCAD, librecad.org, is a 2D CAD program original created to drive a CNC machine. The latest version is dependent on Qt5. Qt has flopped between free and commercial, with a current move back towards commercial. For the next few years, this might be the easiest way into 2D CAD.
2.2 is the latest version of LibreCAD. Linux Mint has a download for LibreCAD 2.1.2. The source code is C++.
FreeCAD, freecadweb.org, is a good place to start. 3D CAD with the option to produce 2D versions. There is an AppImage download for Linux plus versions for those ancient proprietary operating systems, Windows and Mac OS.
FreeCAD has a range of add-on modules you can install direct in FreeCAD. Part of your evaluation of FreeCAD should include those add-on modules. Some of the add-on module names are AirPlaneDesign, BIM, Curves, and Glass.
The FreeCAD download is 236 MB but it contains working examples to help you learn. The FreeCAD AppImage download works reliably and the range of examples is so good that I would be tempted to recommend FreeCAD as your starting point if you are thinking of using 3D somewhere in the future.
People with CAD experience recommend FreeCAD for 3D but not when you only need 2D. For pure 2D, LibreCAD is easier.
There are some simple applications for things like kitchen design. When you outgrow that level of application, you look at LibreCAD and FreeCAD. If there is something specific they do not do, look at their add-on modules. If you are still looking for something specific, look at the packages listed next. Along the way, visit the user forums for the application closest to what you want and post questions about your specific need.
VLSI chip design using the CAD tools from Alliance. www-soc.lip6.fr/equipe-cian/logiciels/alliance/
BRL-CAD, brlcad.org, has 35 years of development by the American Department of Defence and is now open source. BRL_CAD has more contributed code than any other CAD package. The quality and licensing details vary. This is the only CAD that runs on odd operating systems like Irix.
The download is 280 MB so research BRL-CAD before chewing up bandwidth with a download.
Electric is CAD for circuit design. www.staticfreesoft.com/productsFree.html
Design printed circuit boards, PCBs, and display what the finished board might look like. www.kicad-pcb.org
LeoCAD, www.leocad.org, designs 3D models you can construct with Lego bricks. There appears to be no other use for LeoCAD.
OpenSCAD, www.openscad.org, builds 3D models from a script. You have to learn the OpenSCAD scripting language. OpenSCAD is used by people who want to generate the script using other software.
If you program using Python and want to produce CAD output, look at https://sourceforge.net/projects/pythoncad/.
QCad is moving toward a professional version and limiting access to only the source code for the community version, not compiled downloads. You might find LibreCAD will last longer as free software.
SagCAD is 2D CAD from Japan, http://sagcad.osdn.jp/. The last release was 2009. The manual is translated into German and into English (by a machine). The images do not show anything that is not in some other CAD programs.
SolveSpace, http://solvespace.com/index.pl, builds 2D and 3D models from parameters. The parameters could be generated by another program.
Sweet Home 3D
Do you want to design a house? Download Sweet Home 3D from www.sweethome3d.com or view the source code at sourceforge.net/projects/sweethome3d. You design in 2D and the software presents a 3D view. You can add furniture and alter their dimensions. Things like windows can be edited to have an elevation. Add colours and textures.
The software stores designs as SH3F format files and imports furniture in OBJ, DAE / Collada, KMZ or 3DS format. The software is created using Java. You can download a version without Java or, to avoid all the Java version incompatibilities, you can download a huge file containing the software plus a compatible Java.
There are some furniture models included and you can download thousands of other models covering everything from completely useless (An iPod) through to a pair of scissors. There are several variations of most things, power sockets, lights, staircases, and there are decorative accessories including pizza. Think about designing the size of a kitchen bench then placing various food and utensils on top to see what that size benchtop would really look like when in use.
Design model train railways using Xtrkcad from www.xtrkcad.org.
AutoCAD is the Microsoft/Apple/Chrome locked up proprietary expensive CAD used as the standard tool in some industries. If you are a student headed into one of the locked down industries, you can get AutoCAD free while you are a student. Experienced people talk about autoCAD being too hard for beginners. They recommend you start learning CAD using a simpler program.
LibreCAD for 2D and Free cad for 3D. After that, if you are still looking, you might need a commercial product or a discussion with people who already produce designs similar to what you want to produce.