Slack is the teams primary method of communication, it is essential that everyone on the team check slack for messages at least once a day. You should read all the unread messages in all the channels to which you are subscribed. If you use a mobile phone the easiest way to do this is to add the Slack app to your phone and set up notifications.
If you read a message that is relevant to you, you should use the “Add reaction” button (smiley face with a plus) to add an emoji to the message to acknowledge that you have read the message. One of the check_mark emojis are good for this. Since not everyone is reading Slack all the time it is impossible to know if someone has read the message yet. The check mark lets the sender know if you have seen the information. If you have a reaction feel free to use a different emoji that reflects your reaction. The check_mark is a handy way to acknowledge reading it without committing to a reaction.
It is best to respond to a message in Slack by creating a thread. The first reaction to reading a message is to just post a new message in response, but more than one conversation can be going on at a time and the messages can get mixed up. Also, it makes it easier to refer other to the conversation later by just referring to the post that started the whole conversation.