is this a strong thesis statement?

Legalizing marijuana has many benefits such as, reducing overcrowding in prisons, helping create jobs for low to mid income individuals, reduce the budget for ”the war on drugs” and can treat different medical problems. Legalizing this drug can also increase revenue within the United States and other countries that consider this drug to be an uncontrolled substance.

No. It's FAR too long, and it's more than one sentence. Statement = singular = one sentence.

Your thesis statement must include factual information (which you already have) plus your position/opinion/stance. Without your position on the topic, it isn't a true thesis statement. So think of this sentence as the angle you want to take on the topic and what you intend to prove by the end of your paper. (If your statement is simply factual, then there's nothing to prove!)

http://blog.eduify.com/index.php/2009/06/21/5-tips-on-how-to-write-a-strong-thesis-statement/
Read carefully and follow ALL directions.

http://leo.stcloudstate.edu/acadwrite/thesistatement.html
This is one of the very best places I've seen online to help students write good thesis statements. It shows you sentences that aren't thesis statements and how to turn each one into real thesis statements.

would this make a good introduction?

What is your absolutely basic idea there? What do you want to prove by means of your paper?

yes it is for the start

Yes, the provided statement can be considered a strong thesis statement. It clearly states the main argument ("Legalizing marijuana has many benefits") and provides several points that support this argument, such as reducing prison overcrowding, creating jobs, reducing the budget for the war on drugs, treating medical problems, and increasing revenue.

To determine the strength of a thesis statement, it's important to consider if it is specific, debatable, and provides a clear direction for the essay. In this case, the thesis statement meets these criteria by presenting a clear argument and offering multiple supporting points. However, keep in mind that the strength of a thesis statement can also depend on the context and requirements of the specific essay or assignment.