Question

When someone needs help, I try to lend a hand.
Is this an idiom?

Answer 1

Yes, that is an idiom. An idiom is a word or phrase that is easily noticed to mean something else. For example, here it is saying "lend a hand" or help someone. Another example is when someone says they kicked the bucket, they are now dead. 

Answer 2

Yes this is because a idiom is a group of words established by usage as having a meaning not deducible from those of the individual words (e.g., rain cats and dogs, see the light). So  saying lend a hand is an idiom (this is because it is figuratively speaking).