Is it good to only read the high-rated journals as a starting PhD student? I just started my PhD in computer science (first year) and I enjoy reading the high-rated journals related to my field. It is true that I find them, sometimes, complicated (in some sense) and I cannot continue the reading. The reason why I like reading such journals is that (1) there are many beautiful ideas, (2) solid works and (3) I would like to submit to these journals in the future.

I think, however, that this strategy is not quite good as a junior (student). May be I should read other conference papers and journals.

Can you please suggest some useful advice.
I've never been one to sit down and read a journal edition cover to cover. I think you're better off skimming titles and abstracts for articles that pique your interest. You should do this for conference proceedings too. You can't attend every talk at a conference, nor should you really expect yourself to read every article. 

At some point you will need to narrow your interest to something pretty specific in order to develop a dissertation. You will also need to develop depth and historical perspective on your chosen topic. You should probably spend as much, if not more, time reading interesting individual articles and all their important references (and so on!) as you should trying to read whole journal editions or entire conference proceedings. There's value in a depth-oriented approach to reading articles. 

I find that there's a pruning strategy of sorts that keeps you out of stuff that's not interesting. Sometimes randomly selected articles can be worthwhile, but there's lots of stuff out there that will be boring to you. Using a good article as a guide and reading some of its references can be much more productive.