"You need a lemon."
Translation:तुझे एक नींबू चाहिए।
The चाहना means 'to want'.
However, the form चाहिए is special and can almost be treated as its own word. When used with nouns, it expresses a 'need' or stronger form of 'want'. When used with verbs, it expresses modality ('should' or 'need to').
तुम एक नींबू चाहते हो। - You want a lemon
तुम्हें एक नींबू चाहिए। - You need a lemon (Or 'You want a lemon' conveying more urgency/intensity than the other sentence)
मैं जाना चाहता हूँ । - I want to go
मुझे जाना चाहिए ।- I should go
तू/तुम/आप - 'You'
तुझे/तुम्हें/आपको - 'To you'
The 'to you' form should be used with चाहिए and the sentence literally translates to something like 'To you, a lemon is needed'.
Note: As mentioned above, तुझे is a form of तू and is hence the least formal version. You should only use it for very close friends and family who are of the same age as yourself. The तुम form तुम्हें should be used for everyone else who is of age or younger:
तुम्हें एक नींबू चाहिए