You shouldn't wear polyester clothing.

You should wear organic cotton.

​Should

You could try to reduce how much plastic you use.

You can start by not using straws!

Can/could

Offering Advice

​Must/Have to

You must not use plastic single use water bottles.

You have to get a metal reusable one.

This is more friendly, the meaning is more suggestive. The speaker is simply

This is slightly stronger, the speaker believes this is what the other person should do.

This is the strongest form of suggest, and the speaker is telling the other person what to do.