There is no shortage of Sokoban implementations. One of the most famous implementations is known as Sokoban YASC. The acronym stands for “Yet Another Sokoban Clone.”
The basic concept of a Sokoban level is quite easy to understand: you move the boxes so every box is on a goal.
Figuring out the specific steps to solve a particular level, that can be quite difficult, especially if there are some boxes already on goals and you must move them out before you can move the others in.
Sokoban levels are often grouped into sets. Presumably the first level of a set is easy. If you solve it, you go on to the second level, which is perhaps slightly more difficult.
If you like, you can refer to my GitHub repository. To enable you to play the game in your Web browser without having to pull the repository, it has a gh-pages branch.