Here's how you would use it in a sentence:
* "Tu es mon seul(e) amour." (You are my one and only love.)
You can also use more poetic expressions like:
* "Tu es l'unique personne que j'aime." (You are the only person I love.)
* "Tu es la seule qui compte pour moi." (You are the only one who matters to me.)
* "Tu es le seul/la seule qui me rend heureux/heureuse." (You are the only one who makes me happy.)
The choice of expression depends on the context and the level of formality you want to convey.