Build a portfolio of your work by being a volunteer. Find out local organizations that are of interest to you and get involved. You don't have to devote all of your time, and you can even do more than one. It will get you more experience as well as a good mark on your resume. There are many resources online to help you find what you are looking for.
Since you will be working closely with the public, another helpful tip is to make sure your speaking skills are in top working order. Being able to represent yourself is just as important as who you work for. You are the person who represents the company, and if you have problems dealing with critics, answering questions, and giving a rich, informative proposition, your company suffers.
Other important things are flexibility, getting the project done in a timely manner, and working with different people. Not everyone is going to share your enthusiasm right away. What you say, how you project yourself, familiarity, and urgency, are all important so that people are persuaded to see things the way you want them to. A really good PR person is one who can anticipate problems that arise, and take pro-active action. The more active you are rather than reactive, the easier it will be to make your clients want what you have to offer. Giving them choices also makes them feel that they had a hand in the decisions that are made.
No one will rise to the top over night. But if you use these tips, as well as any other that work for you, you will find that you will accomplish more, and have a better reputation than most. When you have become a familiar face, or have got high marks from others who have dealt with you, you may find that companies will begin to seek you out. They will want someone who can promote their company in the most professional and believable manner.