We know that in japanese people usually say '愛している', TO LOVER. CAN i say 愛する? or あなたがすきです?. Besides, is '愛している' is a single verb?
Since the question regards "expressing love" I feel compelled to add something.
It is very important to underline that "expressing love" in Japan is a concept that can be very very different from the way we are used to see it "in the west" (to make a rough generalisation, say mostly Europe and North America).
Based on both personal and friends experiences in regard to "love" in Japan (and I'm talking of several cases, both "foreigner-Japanese" and "Japanese-Japanese") saying "I love you", or 愛している, is not so common after all. Of course, this is something that might greatly depend on the people, but I have to say that in general is as I said. Also, my impression is that if it is being used, is mostly because Japanese people now know well that foreigner like to say "I love you" and that is translated as "愛している!"... I hope what I mean here is clear, it's a bit hard to explain (and besides, as I said is just my personal feeling).
Anyway, you will hear much more often Japanese people saying (あなたが)好きです or 大好きです rather than 愛している, even when the relationship has been going on for a while. This also is quite interesting as it gives a different perspective to the word "好き", that is often simply translated as "to like", while sometimes might bear a much deeper meaning, depending on the situation.
But I have to say that most of the times... you probably will not hear anything at all! My feeling is that in "the west" we say "I love you" or in general we express our love through (spoken or written) words much more often than Japanese people (in general) do. And probably when and if they do they do it in a much different way anyway.
I guess that this (feelings) is one of those topics where if you start digging you realise that sometimes even the word "translation" becomes almost meaningless. You can find the right words, you can find the most suitable words ever actually to express what you have in mind, or in you heart for that matters. However, no matter how perfect your translation is, you cannot change the way people feel. That's where sometimes communication with words fails, big time. Fortunately, that is not the only form of communication there is. :)
This could probably start an interesting discussion about the different way in which people in Japan (and I believe in good part of Asia in general) live and express their feelings, especially about love. However, I believe that this would go too much off topic.
I hope what I tried to express is clear, is a very subtle topic and I hope I did not create misunderstandings. If unclear, please leave a comment.
愛する sounds like "I will love you", but not "I love you now". So, 愛している is correct.
If you're close enough to say such a thing, you can change している→してる. It sounds more relaxed!
You can say 「[other person's name]のことがすきだ」 if you're a guy, or 「[other person's name]のことがすき」 if you're a girl. You can replace the name with a word for "you" like きみ or あなた if you want, but my old girlfriend didn't like to be called by a pronoun, even by me or her own family.
愛している is a noun meaning "love" plus a form of する. There are a lot of nouns that can be used directly with する.