do you think i could start reading Ultimate Spider- Man, starting at #9 without having to read any other past Ultimate comics and only follow this Ultimate Spider- Man from issue #9 and on? 

I've read most of the early Ultimate spiderman. But I can't remember which story arcs starts at which issue. What happens in issue 9?

I'd recommend starting at the beginning with trades as they are worth the read. But I think with the transition between the ultimate line you should be fine.

If you know anything about spider-man already you can skip the first 8 issues. The first arc is pretty much Peter getting his powers and dealing with it if I remember correctly. Sure, it'd be nice if you had access to the whole thing but 8 issues won't change much, Ultimate Spider-Man ran for a loooong time.  
 
Also; I pretty much only read Spider-Man from the Ultimate Universe and it was fine.

I think OP is talking about Ultimate Comics: Spider-Man #9, as in the issue that just came out last month.

its true, he is talking about ultimate comics. ummm...  i think if you keep buying it you could get drawn in and know enough, especially if you keep checking comicvine haha 
 
but ultimate spiderman is really awesome, and id definitely recommend reading it

I am in love with the Ultimate Spider-Man Title... so I suggest starting from the beginning  and not the one from last year... I mean the one that started in 2001!