Yes, exactly, fake your confidence!
Faking it, otherwise known as "the fake it until you make it" school of confidence building, is a form of positive thinking. When we think positively, the hormones pumped into our bloodstream may actually help the brain think more clearly. They also squelch fear, which can block clear thinking. So this mindset really just means, don't let any fear or uncertainty about this new role get in the way of just giving it your best shot and learning along the way.
Then, with this positive mindset, make sure you are prepared with all the training possible. (If you always try to fake it, without gaining the actual knowledge and tools it takes to do the project manager job, you will
eventually fall flat on your face. Positive thinking only works in the end when we are actually gaining the necessary skills along the way. Ned Herman, in his book, The Whole Brain Business Book explains this process further. Read, ask questions, take a class—whatever you can fit in to learn about what you're supposed to do in this role. But also learn everything you can about what could go wrong, what to watch out for, what to pay attention to. Lack of confidence is often borne of fear of the unknown. Plenty of people have experienced the good, the bad, and the ugly on their own projects, and lived to tell about it. And they often are happy to talk about it, to save fellow PMs the same pain. So reduce your own unknowns by reading articles or blogs online from practicing PMs, finding more experienced project managers in your own company to take to lunch—whatever it takes to get some first-hand knowledge of typical problems and solutions. And then actually learn, prepare, and plan to use what you've learned on your own project.
Is it OK to admit that you do not have all the knowledge? Yes, it is OK. In fact, it's insane to expect someone new to the role to know everything. You could even lose credibility if you are not honest about your knowledge. This does not mean, however, that you can't have an opinion and express it as you hit new situations and problems on your project. Just be sure to make a distinction between what you know vs. your opinion on a subject and what facts you have based that opinion on. The best teams know that their project universe is complex, and no one has all the answers about the best way to get all this done. Honest, open communication boosts your credibility with your team and opens the door to collaboration with them, especially if you show a willingness to learn and change your mind when you are shown a better way.