Good plan - start with a few people you can trust, so that later on, if you tell more people and someone is a wanker about it, you've got the perspective of "Oh, but it's just them, lots of other people are fine with it." Because people who are transphobic often try to paint their personal reaction as being a General Response instead, so it's very important to have good allies to remind you that not everyone thinks the way that person does.
And just be comfortable in yourself, because I've found that people respond very positively to anyone who's obviously being their true selves. Figure out what kind of woman you are, and be that woman (and let it change if it wants to) - don't be pressured by cis- or trans-peers into trying to meet some expectation of What A Young Trans Woman Should Look Like™. You can be a princess, or Tank Girl, or Shane McCutcheon, or whoever you really are. There are billions of kinds of woman, you get to be your kind. The people at the gender clinic will also sense when you're being you, and respond well to that.
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