Hi,
In general there are two main ways to get a physical object into a 3D environment.
1. Model it
2. Digitise it (3D scan)

In case you don't have a digitiser, and given the simplicity and geometry of your sample knife, your fastest option is probably to model it (Option 1).
Depending on your proficiency with cad tools most systems have a built in import system for using images to help create the models.
You might need to save the image as an svg file to use it in tinkercad (but check as I am not sure).
If tinkercad isn't working for you I suggest you take a look at Fusion 360, it is free for non-commercial use (or even commercial given certain conditions).