Look inwards for a moment
When you feel jealous, don't be quick to only look outside for the faults or for what your partner could have done wrong. Ask yourself if for a moment it could have been because of your insecurities buried deep down somewhere.
If your partner is hanging out with someone who you think could be a threat to you, probably because you think they're richer, more beautiful, or better than you, I think it might be an insecurity you have, and not really about them. When you feel more comfortable about yourself, love yourself more, value and treasure yourself, you will find out that you stop being worried about people hanging around your partner.
Don't play the Blame Game
Your feelings are yours, and as much as they might have been triggered by an action of your partner, a friendship they have, or people around them. So, if you turn around to blame or frustrate your partner because of your feelings of jealousy, you might be destroying your relationship with your own hands.
Don't nag or fight or lash out and hurt your partner in moments of jealousy, because by the time your emotions have run out, you would have an angry partner or perhaps a broken relationship.
All the actions you should take in your relationship must be constructive and objective, and not influenced by emotions because when you're emotional, you are bound to make the wrong decision.