Hello! It’s awesome that you’re already applying to internships this early on. Since STEP is geared towards freshmen and sophomores, don’t worry too much about knowing anything advanced.
For the technical part, if you’re not used to leetcode the only solution is to practice. Leetcode difficulty they give is usually easy-medium for STEP so I’d recommend practicing these levels. For instance, you can check out lc64 (minimum path sum), lc 200 (Number of Islands), lc108 (Convert Sorted Array to Binary Search Tree), lc125 (Valid Palindrome), Number of subarrays having sum in a given range - GeeksforGeeks to get you started. As for DSA, review concepts such as lists, arrays, loops, sorting, complexity etc since they will be asking about those as well as follow up questions to the coding part. Depending on your background, some interviewers may also ask DP or recursion type questions but harder algorithms are very rarely asked.
As for the behavioral part, aside from asking about things on your resume (be prepared to answer anything on it), most behavioral questions tend to be about your interest in cs or situational. For example, questions like “why are you passionate about engineering?” or “tell me about a time when you were a leader of a group and you struggled - how did you overcome it?” The best way to prep for the latter type is to come up with a handful of situations that can answer a lot of these types of questions.
I hope this helps, good luck!