I'm having trouble on what to do next after frame advantage in KI any tips ?

The first thing to understand is that frame advantage is designed to force your opponent (not you) to take a risk. If you press your fastest button after you have frame advantage, your opponent has only two real options: block, or try to escape using invincibility (a DP, a backdash, etc), and these options are either quite risky, or often predictable. So if you understand that your opponent is more scared of you than you need to be of him, you can start taking appropriate action.
If your opponent blocks (as should be the case a lot of the time), that's when you are able to use buttons other than your fastest button, because suddenly you don't need the super fast startup to interrupt any of his actions. These "bigger" buttons typically are better for mixups (they can sometimes be overheads, etc), but they have to be "earned" in a sense... you have to convince your opponent, who might not even know he is at frame disadvantage, that he needs to sit still, because trying to move around is going to get him killed.
This is why I would advise you, especially in online ranked matches against a random player where you only get one game, to almost always press your fastest button after you earn frame advantage and cancel it into an opener so you can do a combo if it hits. If your opponent tries to jump or press his own button, you'll win... if he tries to block, you'll earn some meter and do some chip damage (which is still a win), and if he tries to hit you with a reversal, you'll probably lose, but you'll gain information that you can use for later. If he tries to backdash, depending on the special move you cancel your move into (Jago's wind kick, for example), you'll probably hit him anyway!
You've probably seen good players "lock opponents down until they get a hit" (Grimmmz is particularly good at this), and this is what you're trying to emulate, but you'll notice that they rarely swing with "big buttons" first... they force the opponent to block small, fast buttons for a while, earning small bits of damage here and there, until the opponent finally gets it through their head that they simply can't press buttons. Once they notice their opponent start to block, they start using better mixups that would otherwise lose to an opponent who is mashing buttons, but they had to *earn* this opportunity first. And if their opponent never learns this lesson, then they win using their fast buttons and move on to the next match.
Lastly, don't be afraid about getting hit by a DP in this game so much. If you notice yourself getting hit by a lot of DPs, then go ahead and try to bait one out after frame advantage (or better yet, do your own invincible DP to interrupt it and instinct cancel), but reversals usually don't lead to huge damage in this game, so you can accept getting hit by one or two. Every time your opponent DPs, he's telling you he's so scared of your pressure he's willing to take The Big Risk. This is good for you long term.