@RawadM — much respect to you for being eager to do this and being so motivated to become a freelances. But in all honesty, if you’re not at the point where you know how to change an existing (fairly simple) WordPress theme (or how to figure out something in the footer is controlled by widgets, in this case), I really think you’re not at a point yet where you’re ready to work on a site for a client.
Of course, if you do, then you’ll learn a lot (probably by making mistakes), but doing that while working for a client is not the right approach. A client’s site should not be used as your playground/testing grounds.
I strongly suggest you follow @shaneisme’s advice (read the book he suggested and look into child themes), and practice a lot more before you take on the responsibility of doing a client’s site.
If course, it’s all up to you to make the choice and I applaud you for your enthusiasm. But be aware that if you end up not delivering top quality work, the client will not be happy and instead of referring you to other clients, they may actually tell people to not hire you.
I wish you all the luck in the world going into the world of freelancing, but you would be most successful if you don’t try to run a marathon when you just learned how to run a 10k.