How to say I love you in Korean

"SARANGHAE" 사랑해
"SARANGHAEYO" 사랑해요
"SARANGHAMNIDA" 사랑합니다
"SARANGHANDA" 사랑한다

Those sentences, I mentioned in different ways, are almost the same. They just differ when to use regarding who you speak such as older or younger people and someone you are not that close to. 

SARANGHAE is for younger people and your close friends. 
SARANGHAEYO and SARANGHAMNIDA are formal ways to say, which means you will say to elderly and someone you are not that close. But actually, you would not say "I love you" to strangers in anyway, so in this case, the person should be the one you know well

SARANGHANDA is kind of informal way to express. I think it is used to emphasise your love.

It is right time for you now to tell someone that you love him or her in Korean!