Some bugs are labeled "HelpWanted":
https://bugs.chromium.org/p/v8/issues/list?q=label%3AHelpWanted&can=2They probably vary a lot in terms of required effort and difficulty.
The label likely hasn't been applied exhaustively, so you can also look at other open bugs. Though keep in mind that many of the older bugs are open for a reason: either they were considered unimportant, or we couldn't think of an acceptable solution, etc. But I'm sure there are some that would benefit from some love :)
Feel free to pick something you find interesting, whether that's performance work or spec compliance issues or engineering productivity (like faster builds and tests) or embedder documentation/samples, etc.
Note that ramping up on V8 is a serious time commitment. Don't get discouraged when it takes a while (and a lot of reading of docs and code) until you feel somewhat familiar.