How to Get Someone to Tell You a Secret
To get someone to tell you a secret, calmly ask them to tell you. Say that you want to know so you can support them. Then, assure them you'll keep their secret. However, if they don't want to tell you something, don't push them, since this will make them uncomfortable. If you want someone to tell you more secrets in the long-term, make sure you keep their secrets so they'll trust you in the future. You should also share your own secrets with them, which will make them feel more confident opening up to you.
Enjoy sincere conversations. Have conversation with the person that are sincere and cover a range of topics. Talking about meaningful things to the person can help them consciously connect positive feelings about you and make them more likely to open up to you. Incorporate different topics into your conversation. Keep it lighthearted and funny at times and balance that with more serious matters at others. Be as sincere as possible without appearing fake. For example, if your friend having trouble in your relationship in a way you cannot relate you can assure them and say “I don't totally understand how you're feeling, but I am happy to talk about this however much and in whatever way you need.” This would be better than simply saying “I know exactly how you feel,” especially if you don't Listen intently. Listen closely to what they are saying during your conversations. Reference what they've said other times you talk, which shows to them that you are paying close attention to them and their needs. Ask questions during the conversation, which can show that you're listening closely to what your friend is saying. Notice differences in their tone or demeanor, which may indicate they are hesitant about something or it is difficult for them. You can gently probe by asking “is everything alright?” This will give them a chance to let you know if anything is wrong. Don't push your friend to tell you anything with which they are uncomfortable. This shows them that you are trustworthy and genuinely there for them. Talk about yourself, too. Make sure to talk about yourself during your conversations. Give little bits of information about yourself as you feel comfortable, which can in turn put them at ease and show that they can confide and trust in you. Discuss different topics with your friend/family member including serious and lighthearted matters. This can help them see your similarities and may make them more likely to confide a secret in you. Sharing can really help establish a relationship of trust between people, especially if both individuals are comfortable talking about themselves. Keep what you talk about and share commensurate with what your friend/family member shares. Oversharing and not sharing enough may undermine you ability to get them to tell you secrets. Receive them unconditionally. An integral part of building trust in any relationship is accepting the person unconditionally. This may make them more likely to divulge secrets to you. Don't push or even force them to do or say something with which they arn't comfortable with. Assure them in any way you can. For example, if you know she isn't telling you everything and is keeping secrets, you can say “You can tell me anything, you know that right?” Be reliable. Make sure to be there and honor commitments with your friend. This shows them that they can rely on you at time, including telling and keeping secrets. Follow up on any commitment you make to the best of your ability. Showing that you are reliable even with the most basic things helps maintain trust. If you cannot honor something, give the person plenty of notice and make to explain the circumstances and then apologize. Make sure to not pass judgment on anything they tell you. This can help them be more ready to confide in you. Show your independence. In conversations and interactions, show your friend that you are independent and able to think for yourself. Demonstrating that you are not susceptible to outside opinions or pressure to talk about them can help show that you are reliable, sincere, and trustworthy. This may make them more likely to tell you their secrets. Don't repeat gossip you've heard about them or other people. If you do, this could make them wonder what you say about them when their not around. Assert you opinions without being overbearing to show your friend that you can hold your own with others and are not susceptible to influence by outside forces. Maintain discretion. Likely the best way to gain the person's trust and get them to tell you secrets is maintain complete discretion on all matters they discusses with you. Avoid divulging information they've told you to others or making light of it in other conversations you have with them. Keep anything you think might be sensitive to your friend to yourself. Ask if you are not sure if you can say something to others, but know this might undermine their trust for you. For example, if they tells you there is pregnant, just ask “should I keep this to myself or are you sharing the news with others.” Respect there wishes with whatever there decision on relaying information is. Ask the person to divulge. If you know your friend has a secret, just ask them to tell you. No matter their decision, accept and move on from it. Ask in a patient and non-confrontational manner. Assure the person you want to know because you support them and that you will keep the secret to yourself. Support the person. Many people who harbor secrets may be in trouble or feel like they are beyond help. Ask them if you can help and then support them in whatever way they need and want. Research shows that holding on to secrets can cause physical and emotional distress. Tell your friend that if they want to talk to you about their secret it may make them feel better. Stay silent. No matter the temptation—if the secret is very happy or very serious—do not divulge the person's secret to any other individual. They likely have a good reason to keep the secret and may only want those closest to them to know what's happening. Avoid all temptation to tell the secret, which can have serious consequences for your relationship. Speak to an authority figure. If you feel the need to divulge the secret because of its serious nature such as abuse, an affair, or health issue, speak to a professional or someone in authority. Make it clear that you need advice on how to handle the situation. Avoid using the person's name or too many details that could indicate the person's identity. Consider speaking to a professional such as a lawyer or police office about the secret and how to handle it. Discuss your feelings with someone who doesn't know the person who has divulged their secret. Share your own secret. If your friend shares a serious secret with you, consider reciprocating and sharing one with them. Knowing that they are keeping your secrets may make you more likely to hold on to theirs as well. Make sure the secret you share is similar to theirs. Remember it's not a competition, but meant as a measure to assure both of you. Say, “Please keep this to yourself, but I understand how you feel. I have a secret, too.” 
Enjoy sincere conversations. Listen intently. Talk about yourself, too. Receive them unconditionally. Be reliable. Show your independence. Maintain discretion. Ask the person to divulge. Support the person. Stay silent. Speak to an authority figure. Share your own secret. 