A generalized question can only get a generalized answer.
The most significant accomplishment is validating that the product you have built is a fit with your target market. This is demonstrated primarily by engagement (the people who sign-up or who previously visited, continue to return) and secondarily by growth, ideally based on word-of-mouth or viral growth but effectively converting paid traffic is a great second prize.
Other significant accomplishments include:
Not running out of money
Recruiting and retaining great talent who believe in the founders' vision.
Your loved ones not thinking you're as crazy as they thought you were a year ago.
I'm happy to talk to you in a call to give you more specifics about what you want to set as your goals more specific to your startup.
The first year for me is the year to prototype as much as possible. Do the MVP (minimum viable product) with quality and track down analytics to the user engagement.
Start with the core idea and build out worth. The less features the better. Make the essential and remove all non essencial features.
The goal for the first year is to prove that there is a market/need for your idea. If it doesn't grow on it's own, then probably you need to tweak it and adjust your initial idea.
Users should bring users. Users should come back. If that doesn't happen, then probably you are working on something that can make all the logic in the world but that users don't want or need.
In a nut shell, the first year is trial and error. Analytics, tune up, change, try again, until you reach a consistent growth.
My name is Mijael Feldman, im co-founder of BOMBAcamp (www.bombacamp.com), an early stage startup accelerator for spanish speaking market.
How many paying customers you have for your product/service.
Visits/users/suscribers/followers are worth nothing if you dont have people paying for you product.
Best validation for a startup in its first year is to habe customer paying.
Validate a problem -> validate the solution
-> test solution -> paying customers. Dont worry about getting a million customers, focus on getting the first 10, then 100 then 1000.
Hope it helps! If you want more details of how doing this process i am open to jave a call with you.