Eh, there is probably a lot I could try saying. But, I guess most of it requires more experience than you might currently have. For example, if someone is repeatedly rapid jabbing you and not letting up you ASDI/SDI out in some respect, then use the proper according move. Say I am getting Captain Falcon and Sheik rapid jabs me. I ASDI/SDI away from sheik, then get an Fsmash pulled. The reason is that Fsmash would have higher ground priority over rapid jab and go through the hitbox of jab. Or if I get jabbed by some character like Fox, then I Nair out since jab -> w/e from fox shouldn't be a solid follow-up I believe.
Pretty much everything in this game has a counter in some regard and you just need to apply the right countermeasure for whatever is giving you issues. The normally the simplest method is to just wait. Wait, outside a person's attack range waiting for them to whiff the attack, then you rush in and counter it. W/e the attack or commitment was from them.
It'll just take practice and time to figure out the right counter for whatever is giving you issues. Oh, but the biggest thing is once someone makes a mistake and you get the chance to punish them for it, figure out how to keep the punish going on for as long as possible.