Let's start with how to identify a thesis statement, since that will include a definition.
A thesis statement is a claim made by the author at the beginning of the work. It should express the author's stance on a given topic and give a sort of direction for the entire essay or piece. You can often find it near the end of the introduction or intro paragraph.
In terms of writing a thesis statement, you can use a couple of different strategies, depending on how you think. Before you try either of them, it is important to be well-read on your topic, having already explored your sources, whatever kind of sources they may be.
1) You could start with a stance. For an oversimplified example, "Summer is better than winter." Once you have your stance, you can come up with things that support that stance. "Summer is better than winter because it is easier to travel and there is more sunlight." Depending on the requirements for your assignment, you might try a three-part thesis. To return to our example, that would mean there would be three reasons why summer is better than winter, and each body paragraph would discuss one of those things particularly. If you are an upperclassman in college, the three-part thesis is probably insufficient, in which case you should work on developing a more complicated stance that can be broken down over many body paragraphs.
2) Another way to go about writing a thesis statement would be to work backwards. For example, perhaps your topic is "season,s" and you know you want to talk about the length of the days, the temperature, and the activities available during summer. Using these pieces, you can construct a stance that your pieces would support. It just depends on whether it is easier for you to work from an overarching claim and then come up with supporting evidence or to work from the specific things you want to discuss and determine an argument that knits them together and synthesizes them.
I have linked you to a resource that discusses writing the thesis statement at length. Good luck!