There are no rules that tell you what you can or can't think about during meditation. However, you don't want to force the thought. Remember that when you meditate, you are allowing your mind to be free. It's a chance to feel each emotion that comes up, in a safe space, that helps you analyze without judgment or worry.
Meditation is not about talking to yourself or having a conversation in your mind. It's learning to listen in silence, & paying attention to your breaths, while allowing the emotions to happen naturally.
"Enlightenment is when a wave realizes it's the ocean."- Thich Nhat Hanh
In the beginning, your mind will feel like it's racing. You're doing something new and this might make you a little hesitant about what is in your thoughts. You may hear your inside voice question the purpose of meditation. Anger may bubble up from an argument, sadness from a loss, or it may trigger overwhelming emotions of a peace you've never felt before. If it makes you cry, sit with the tears. If it brings happiness, sit with the smile. If it beings anger, breathe deep and slow, and let it release.
The voice in our head can get noisy sometimes, don't get frustrated on the days it wants to shout instead of listening to your breaths. The more you practice though, you will finally begin to see within; a type of vision many rarely get to experience without learning meditation. Your mind will take you to place of calm you may not recognize and you'll wonder if it's real. A feeling of bliss so comforting, it goes beyond mere relaxation. All the negatives that try to steer your life into chaos will suddenly not matter. When you are in this sublime state, it will be as if the turmoil floats above you, where you can still see it, but you're no longer grasping at it.
To reiterate, just the questioning of 'what should I think about' isn't necessary. You don't have to plan what you should have in your mind during meditation. There's no rehearsal and no preparation of any words to focus on. This is your chance to just be there. This is your opportunity to stretch your mind into acceptance.
Full disclosure, I am Buddhist. We have mantras and prayers that we use prior to and after our meditation sessions. But it's not necessary or required for meditation. Meditation spans beyond specific theology. If you choose to, you can use a mantra to settle in to your practice. This can help calm your mind further. Any mantra is fine; some people use mala beads or counting beads to help them count the number of times the mantra is repeated. This method of repeating can be used to train your mind to settle.
Meditation is a place of discovery. What will you find there, as you push your problems away from your mind to make space for self?
I'm glad you're here. -Quin