So, you have been fascinated by the incredible number of mechanisms and models that can be assembled from LEGO Technic.
It is best to buy one of the donor sets that have a wide variety of parts from which you can build not only what is indicated in the instructions, but also many other models.
Model options can be found at rebrickable.com
The most useful set is LEGO 42082 - Rough Terrain Crane
This big red crane is considered the most optimal set as a starter kit, it is of course quite difficult to find now because it was produced for a long time, but if you succeed, this set will be enough for quite a lot of different models.
Just look at what options are offered on rebrickable.com
If you are interested in a certain custom model, but you still lack parts, it is best to use the site bricklink.com to purchase the missing ones.
The site has a very convenient and rich functionality.
You can generate a list of parts that you need to build a specific model, specify the parts that the kit you have.
The site itself will select a list of missing parts. You can also specify the country where you need to deliver the parts.
You can specify the seller from which country you want the site to show the availability of parts. Sorting, ordering, shopping cart - everything is very convenient.
The best way to get started with LEGO technic is to learn different ways to transfer power, learn how gear ratios work, what types of gears connect to each other, how to transfer power at an angle by turning 90°, how to change the direction of power transfer. With the help of LEGO technic you can assemble a huge number of different mechanisms to learn the basics of mechanics.
Good luck in the world of Lego Technic.