Formal:
* Я люблю только... (Ya lyublyu tol'ko...) - This is the most literal translation, but it can sound a bit stiff.
* Я люблю исключительно... (Ya lyublyu isklyuchitel'no...) - This emphasizes the exclusivity of your love.
Informal:
* Я люблю только одного/одну... (Ya lyublyu tol'ko odnogo/odnu...) - This specifies that you only love one person.
* Я люблю только тебя/вас... (Ya lyublyu tol'ko tebya/vas...) - This directly expresses your love for a specific person.
* Я люблю только по-настоящему... (Ya lyublyu tol'ko po-nastoyashchemu...) - This highlights that you only experience real love.
Remember to replace the ellipsis (...) with the object of your love.
For example:
* Я люблю только тебя. (Ya lyublyu tol'ko tebya.) - I only love you.
* Я люблю только по-настоящему. (Ya lyublyu tol'ko po-nastoyashchemu.) - I only love truly.
The best way to say it will depend on the specific situation and the relationship you have with the person you're talking to.