Affirmations are positive thoughts that you repeat in your mind. Repeating affirmations to yourself (silently or out loud, your call) can help you combat mental blocks, redirect negative or self-defeating thinking, improve your sense of self-worth, and even calm you down if/when you start to spiral.
This is very much up to you. Here are some (original and borrowed) to get you started. If you Google "affirmations for ----" and fill in what you are trying to accomplish or overcome, you might have even better luck.
I am smart.
I am capable.
I am worthy of respect and forgiveness.
I attain my goals.
I choose my own attitude an am responsible for my own actions.
All of my problems have solutions.
I get better every single day.
I know how and when to ask for help.
I am alright right now.
I can do this.
This is by no means a definitive list, but you are more than welcome to give any or all of them a go.
Any time! They work well if you do them at the same time every day, and then throughout the day if you feel yourself getting frustrated, worked up, or hopeless.