This is something that gets a little iffy in Japanese.
The way of saying 'I love you' in the sense that you mean would be 愛する＝あいする＝ai suru (Change it according to what tense you are speaking in). Now that is just the verb. To say I love you it would be：
The best choice to use here would be the last one, because if you are speaking to the one you love, ususally you have a strong enough relationship to speak casually. The others are actually very awkward.
That being said, Japanese people find it very awkward to say あいしてる. It's too strong and awkward and old. It's usually only used between married couples or older people...maybe. Also, it cannot be used towards parents or friends, it would be awkward. That way of saying it is more of a 'romantic' I love you.
The better choice would be 好き＝すき＝suki.
It means "favorable" or usually used as "like." So yes, it is an adj. In Japanese, liking something is not an action, but a way to describe something.
君（きみ）が好きです。= I like you/ You are favorable.
This is almost always accomidated bt the particle が.
You can also say:
Which are some of the ways of saying "I really like you."
If you are young and not married this is how you say I love you to your friends, families, and the guy/girl you like. If you are confessing though or talking to someone you like in a romantic sense, you usually use 好き or 大好き.
So in short, if you want a less awkward way of saying I love you it would be だいすき and すき if you like someone.
:) Hope I could help.