I like to revise and rephrase these arguments from time to time, so please forgive me if you've read something similar here a hundred times before.
1. Every contingent entity has a sustaining cause. (Premise)
2. Either there is a necessary first cause, or the regress of contingent sustaining causes is infinite. (Implied by 1)
3. The regress of contingent sustaining causes cannot be infinite. (Premise)
4. Therefore, a necessary first cause exists. (From 2 and 3)
This "necessary first cause" is synonymous with the "ground of being," which is God to those of us with a theistic persuasion.