We want to start working in a new research direction. How to proceed as the first student in the group?
For your question from the research groups perspective there is no definitive answer. Many things are possible and it depends on the group, the students, the field and whatnot. However, if you plan to enter such endeavor, you should discuss plans and responsibilities.

For a personal perspective: It is a lot of work to go into a new field. Read basics, but stay up to date. Talk a lot to people who work in the field to get a better sense, what the pressing questions, the important goals are and learn the jargon. Listen to talks on the topic, prepare well ad ask questions to get in discussion.

The bad news is, that nobody can guarantee any short-term outcome (unless you can somehow figure out some low-hanging fruits in the new field)…