Well the most obvious answer is 'I love you', but I suppose there are many ways of showing that we love someone.
Sometimes we use other words, sometimes we use actions instead of words, sometimes we don't say or do anything at all and we let it pass us by.
Personally, if I was in love with someone and wanted to let them know - I would tell then that I loved them, that I treasured them, that I enjoyed spending my time with them and that they brighten up my day.
If it's intimate love, like a relationship, then more romance can be involved. But you can also love someone as a friend or family member. I tell my cat I love him very often for example!