Need help with Star wars viewing order

I have not seen any star wars movie but after i saw the trailer of star wars episode 7 i decided to watch all the movies of star wars but then i found out that many of the episodes are old, so i was wondering if it is ok to watch only few new ones

And if it is required to watch all the episodes than suggest the proper viewing order of the movie

If it is possible than please suggest only few episodes which are mandatory to watch the latest star wars movie!!