My current favorite tool for creation of online games is Construct 2. There are many game engines out there, but Construct 2 is my favorite because of the low cost, and the fact that Scirra (the maker of Construct 2) does not charge a royalty for games you sell or distribute using their engine. Many popular game engines want a chunk of your earnings if you create the next big internet gaming sensation. Scirra is happy to sell you a hobbyist or professional license. That is why you will see Construct 2 favored on this site.
Now, let’s get your first game made with a brief Construct 2 game creation tutorial.