The simplest answer is to ask.
Whatever social media you have, put up a post and request betas**.
Writer friends are a skilled option, but do keep in mind that their schedules are likely to be pretty packed with their own book work. It’s totally okay to ask, but they shouldn’t be your only source of readers.
Some people use friends (non-writer ones) and family (I don’t recommend this and neither do most of the people I’ve conferred with in the past), but you definitely want to get the opinions of people who read your genre and strangers are more likely to be honest since they’re less worried about maintaining a relationship with you.
If you provide a positive beta reading experience then people are much more likely to stay on board and be willing to read future projects for you. This is how you build up your team. Keeping around the betas who read in a timely manner and provide constructive feedback is going to make future work much more pleasant.
Even if you already have a dedicated team of beta readers it’s a good idea to add fresh eyes once in a while. Just because you have willing readers on board doesn’t mean their schedules will align with a project, so you’re likely to have to add to the ranks on occasion.
You can also ask writer friends if they have any beta readers they’d recommend. If you’re a newbie that doesn’t have a writing community yet it can be a frustrating experience. Find useful hashtags and look for opportunities to build your community. Join discord servers and facebook groups relevant to what you write and read. Make friends and communities will open up to you.
**Please be aware that making a recruitment post is likely to mean that the pay-to-read people and the beta exchange people will find you. They’re looking for vulnerable authors who are desperate for readers. I recommend paying beta readers only if you’re not able to find volunteers. While beta exchanges (I’ll read your manuscript if you read mine) aren’t inherently bad, you have to be mindful of your time. If you take on reading a dozen manuscripts just to get beta readers then you’re not going to have much time to write.