See the Linux build instructions.
Look at the Chromium bug tracker for open Linux issues.
Issues marked “Available” are ready for someone to claim. To claim an issue, add a comment and then a project member will mark it “Assigned”. If none of the “Available” issues seem appropriate, you may be able to help an already claimed (“Assigned” or “Started”) issue, but you'll probably want to coordinate with the claimants, to avoid unnecessary duplication of effort.
Issues marked with HelpWanted are a good place to start.
If you think you have discovered a new Linux bug, start by searching for similar issues. When you search, make sure you choose the “All Issues” option, since your bug might have already been fixed, but the default search only looks for open issues. If you can't find a related bug, please create a New Issue. Use the linux defect template.
See Contributing code.
See Linux debugging.