मैं तुमसे प्यार करता हूँ (main tumse pyaar karta hoon) if you are male, मैं तुमसे प्यार करती हूँ (main tumse pyaar karti hoon) if you are female. The verb ending changes with the speaker's gender, not the listener's.
That gender rule catches everyone out. Karta is said by a man,
karti by a woman, regardless of who they are speaking to. A woman
saying karta hoon is a grammatical error, not a stylistic choice.
Softer and stronger versions
मुझे तुमसे प्यार है (mujhe tumse pyaar hai) — "I have love
for you". Gentler, and it avoids the gender ending entirely, which makes it
useful while you are learning.
मैं तुम्हें चाहता/चाहती हूँ (main tumhein chaahta/chaahti
hoon) — "I want you", but in Hindi this reads as deep affection rather than
desire. Common in film dialogue.
तुम मेरी जान हो (tum meri jaan ho) — "you are my life".
Very common between couples and, in a different tone, to a child.
To family
Hindi speakers rarely say pyaar to parents or siblings. Affection is
shown through action and through terms of address —
बेटा/बेटी to a child, भैया/दीदी to a sibling.
A grown child telling a parent "main tumse pyaar karta hoon" would sound
theatrical to most Indian ears.
A note on tum and aap
तुम is informal and correct for a partner or close friend.
आप is formal. Using aap in a declaration of love sounds
either very old-fashioned or very poetic — which is exactly why film songs do
it.
Practise this with a teacher
Reading an answer is one thing; being corrected while you say it is
another. EkGuru tutors teach one to one from
$6
a lesson, and most offer a trial.