Any suggestions on list building strategies for new startups?

There are lots of ways to build a list depending on your product release state, targeted audience, list purpose (sales vs. marketing), goals and how much money you have.

Sign Ups
If you have not launched your product yet, on your homepage or a landing page that provides teasers into what you are doing, you can place a simple sign up for notifications of the release or for requests to be a beta customer.

If your company has a blog, have a place for to people sign up for notifications of blog releases. Make sure they understand that they are also opting-in to marketing emails.

If you collect basic information - Name, email address - you can get expanded information (company, title, phone numbers) through tools like ZoomInfo, or LinkedIn Sales Navigator.

This type of list gathering tends to be well targeted, but low volume. So promoting through social and other channels may be necessary.

Manual
If you know the type of person you are targeting you can find them and email them through LinkedIn Sales Navigator. Or find them on LinkedIn and then use Hunter.io, Anymail Finder or similar tool to get their email addresses. If you have a competitor, go to their webpage and review their customer success stories. By identifying companies and people who already use your competitor you can target those people and also find people like them.

This is cheap, but labor intensive.

Purchase
On the other end of the spectrum is buying a list. If you can define your target market and buyer really well you can get a decent list. If you aren't sure who you are targeting, this can be a big waste of money, time, and effort.

If you provide more details on your situation [Stealth/Product Launched, B2B/B2C, list purpose, sales channels], I (and others) can be more helpful.