Clarification Regarding Web Host, Domain Name

I had just sat down to click on the "buy now" button of a web host service provider when I just had a few more doubts.

I am faced with a few queries like the domain name I need to buy. At the moment, I have not made a proper site for anyone. I am experimenting with a few example sites only. One of my good friends, who is a cricket coach told me that he needs to modify his site. He told me that I can use his site to experiment and learn. So I am now trying to make a site using his images, header etc but with a few modifications. His site contains a home page, a contact page, an image gallery and also a registration page where he wants the students to give their details like name, age, school and submit the details.

Now, I have just managed to have a home page, a contact page and a plain gallery page with a few images. I also made a registration page where the student can submit the details. But I have not been able to test the form on my page to see if it works.

Now this is what I want. I do not have a site of my own now. But I need to have a web host who will give me php support so that I can test whatever page I make. When I am satisfied that I can venture out and make a website for somebody, I intend to start offering my services to my friends (for a nominal fee or even for free if it is going to help me to learn. I do not want to take money for a sloppy site or take my friends for a ride. I want to be able to offer quality service for whatever I charge).

My question now is this. When I need to register for a domain name, is it a good idea to just have a domain name like www.jppp.com ( if it is available or a similar name). Then can I try out the various pages that I make on this site? Once I get my domain name, I need to download a ftp client in order to upload my files, isn't it? Once I am a little confident and offer my services to a client, I make a site on my PC with all the files required. Then do I upload the files to my own domain name to show it to the client for his approval? Or should I have a domain name of my client's choice and then upload it there before making any changes that he wants?

One more question. In my gallery page, I have a few thumbnail images which shows a larger version on hover. Now I can upload the pages which my friend has provided me. But my friend will have many images which he needs to be able to upload himself instead of having to depend on me? I know that I need to use PHP for this. But I do not know the exact method. Can I be shown the way or directed to a tutorial? Will I have to let my client know that the size of the images should not exceed a certain limit? Being a layman, he will not know how to resize images which I can easly do in photoshop, isn't it? What if his images are too many and exceed the amount of webspace that I have?