The two parts to your question involve two different answers. You can find information on how to determine how many cpu minutes the script is using in this Dreamhost Wiki article. This presumes, of course, that you have enabled user resource reporting from the DH control panel (which I assume you have done since you report knowing how many cpu seconds you are using).
Finding completely unique visitors is challenging without implementing special progamming (sessions/cookies, etc), as users visiting from dynamically assigned addresses. proxies, etc. can easily be "miscounted", but you can get a general idea from your existing stats provided by Dreamhost using analog, or install a different stats package yourself if you prefer.
Now I'm confused; First you say you want help to find out how to tell "how many visitors the site has a day" - now you say you "get 10,000 unique visitors a day". Which is it? Do you need help in determining what your traffic is, or do you have that already sorted?
The term "a lot" is relative, but that figure represent over 32 cpu minutes a day and I believe it is "getting up there" for a shared server. PHP.CGI is the program that runs all your php scripts; I have already pointed you to the wiki article on how to track which scripts are using those cpu resources.
CPU minutes are used whenever anything uses the cpu - downloading does use the cpu.
While Dreamhost no longer has a "firm" limit on cpu minutes you may use (search the forum or the wiki for more discussion on this), if your cpu usage starts to impact other users on your server they will require you to take some action.