If you are interested in Justice and Peace issues, but your Parish does not have a group set up, talk to your Parish Priest and see if he is aware of any other Parishioners that might be interested in setting up a small group. Whether there are or not, arrange an open meeting and invite other Parishioners through a note in the newsletter to come along to form a group. Ask the Parish Priest to highlight it; he may like to join as well.
Think about the issues the group is interested in and focus on those. Be realistic in what you aim to do. You could either choose to focus on campaigning or fund raising for your chosen issues - or a bit of both.
You could arrange for a member of another J&P group to come and give advice on setting up a group. CAFOD have plenty of ideas for campaigns you can be involved in and these are often a good start as Parishioners feel happy about doing things for CAFOD.
Don't be put off if you do get minimal support from the Parish. Each Parish is different so it may be best to see what would best suit your Parish. Doing something is better than doing nothing. You may find that forming an ecumenical group is a better option.