The first thing you need to consider is whether you want to have an audio-only baby monitor or one that incorporates video. Some parents choose to use smart home security cameras that send a video feed and alerts to their phones via an internet connection instead. Your choice largely depends on your budget and how high tech you want the baby monitor to be.
A word of caution about extremely cheap baby video monitors (we're talking devices that cost less than $50): they're not known for their security and can be hacked. Be sure to always change the default password of any connected device you purchase. You can also protect yourself by sticking to known vendors who post frequent firmware updates and have easy-to-reach customer support.