At the very first beginning: branding.
Getting the right name for the product, service, or business.
Vision, Mission and Positioning statements.
Logo and corporate identity. Webdesign. Graphic design and copywriting for the different instruments. Et cetera.
That said, you could start with just an AdWords campaign and generate revenue without any branding whatsoever. But the items above is the stuff most startups agonize about in the first few days or weeks.
Then they go ‘out of the building’ and realize all they’ve done so far is plain wrong and redo most of it. Which is fine as you need to learn.
After that, the bulk of the marketing will be doing lots of experiments (growth hacking) using different kinds of sales promotion to drive traffic to the site or service and get a clear picture of the customer journey.
When this is settled, sales promotion efforts will be the order of the day.
The point is that sales promotion can be anything: pr, sea, classical advertising, promo teams, event marketing, speaking engagements, and so on.
So I’d say that the instruments you use in that stage depend on what your initial experiments have taught you.
This explanatory visual about the subject comes from Amir Moin’s great post.