Start with thinking about a topic that is related to the activities you do in your target language. If you read novels or watch films, think about the plot. If you chat with people, think about what they told you, or about what you want to tell them. If you attend lectures, take notes in the target language and learn from them without using your native language.
You may soon find out that thinking about L2 activities in L2 is actually easier than thinking about them in L1. That happens because when you do something in L2, you unconsciously learn new words, and they often don't have exact equivalents in L1. Therefore, it is natural that thinking about them in L2 is easier, even if your L2 level isn't very high.