How can I secure my invention before giving it to a company?

The best course of action would be to file a provisional patent application with the USPTO. A provisional patent application can be filed by you provided that you can write, and use accompanying drawings where necessary or appropriate for the understanding of the written description.
There is no real mandatory format for a provisional application provided that you describe the invention clearly, completely, accurately, and thoroughly. Once you have filed the application, you effectively have senior rights to that invention, meaning that if someone else comes along and files a patent application, provisional or non-provisional, for the same subject matter, you have priority and your patent application will be used against their patent application as prior art. The filing fee for a provisional application is $130 for a small entity, or $65 for a micro-entity - you can probably qualify as a micro-entity since, as you noted, you are an undergrad student. Once the provisional application has been filed, you are free to disclose it to anyone because, as noted, you already have a priority date which will pre-date anyone else with the same invention provided, of course, someone else hasn't filed a patent application before you - the USPTO works on a first-to-file basis - whoever files a patent application for a particular invention first is automatically the senior party. You can then use the provisional patent application to disclose such to this Silicon Valley startup. The only proviso with respect to a provisional patent application is that you need to file a full-blown non-provisional or PCT application within one year of the filing date of your provisional application - if you fail to do so, the provisional application will be abandoned and you will lose your priority date. In addition, without perfecting the conversion from the provisional to the non-provisional or PCT application means that you will never get a patent because provisional applications, in and of themselves, never lead to a patent - the application is never examined and a patent is never issued - the provisional application simply effectively establishes your priority/seniority date relative to others who may be developing the same or similar inventions.