question on picking a domain name

[wxdqz]

I am kind of new to this so I had an idea to test sites first as a sub-domain then if they do well get them their own domain. Also if I picked more of a general domain at first instead of something specific, would it help?

Originally I owned an automotive domain and was going to test off that but I figured something might throw the visitor off and scrapped the domain. Now I am looking to buy another one so I was wondering if my idea might work.

Thanks,
Tony