One phrase, 3 meanings. Learn them one at a time, then test yourself.
Meaning 1 · most common
to heat food again that has already been cooked
Examples
There's some curry in the fridge — just warm it up in the microwave.
Shall I warm up the leftovers from last night?
Meaning 2
To do gentle exercises before a sport or activity to get your body ready and help prevent injury.
Examples
It's important to warm up before you go for a run.
A: Are you ready to play football?
B: Give me a minute to warm up first.
Common mistake
Not: I always warm up myself before running.
Say: I always warm up before running.
Intransitive here — don't add 'myself'.
Another example
Before my PE class, I always warm up to avoid getting hurt during exercise.
Tell it apart
Warm up before the match.
He warmed up to me eventually.
Goes with
Meaning 3
To start feeling more comfortable and friendly with people, especially after being shy or quiet at first.
Examples
It took a while, but she finally warmed up to her new classmates.
A: Was your brother shy at the party?
B: At first, yes, but he warmed up after a while.
Common mistake
Not: He warmed up with us after a while.
Say: He warmed up to us after a while.
'Warm up TO' people (or just 'warm to') — not 'with'.
Another example
When I meet new people, I usually need some time to warm up before I feel comfortable talking.
Tell it apart
She warmed up to the new team.
She warmed up before the run.
Goes with
Read the sentence, then pick the meaning that's in play.
Pick a meaning and say it out loud. Save the ones you mix up.