Everyone is different obviously and you may feel your situation is different because of the type of person your question is about but if you're a human, it's likely you already understand human behaviour so you can probably guess why a person did what they did. If not, ask yourself why you'd do what they did or didn't do/say. Before you ask a question on GAG, check here to see what the general answer is to your question or use the search bar to find similar questions.
"They didn't follow me back, do they not like me?"
Maybe they want more followers than who they're following and even if they did follow you back, it doesn't mean they like you. They may not have noticed someone's followed them either, especially if you've given them the whole of 5 minutes to follow you back. You shouldn't put so much behind a follow online - it means nothing in the whole scheme of things.
"They keep staring me"
This is usually the behaviour of shy people. If they don't stare at other people (some people do just stare in general), then it's obvious that they like something about you or find you attractive. There's also a possibility they're admiring you but they don't fancy a relationship with you or they're looking at you negatively. Only you can work out which one it is because you know more about the person than we do.
"Do guys like sex with fat girls/do they like chubby girls?"
Everyone has a preference. A large number of men will prefer skinnier women, that's just the fact of life but that doesn't mean there are no men out there who don't like chubby/fat women. There's someone for everyone.
"He flirts with girls, is he a player?"
Being a flirt and a player is very different. Flirting is harmless and sometimes meaningless, usually done with strangers who they don't even persue. Players will sleep with many women and lead them on or get into relationships with no intention of being loyal or committed to her. Players use girls for their selfish needs, flirting is not that.
"They keep touching me all the time."
Touching someone non sexually, is a way of connecting with someone and showing interest. It's basic human psychology and that person likely knows about that psychology and are putting it to the test with the person they like aka. you.
"They smiled at me, do they like me?"
Think about when you smile at someone, you might smile at strangers, at friends, family and for all very different reasons. It doesn't always mean you find the person attractive or want to date them. Smiling is usually a way of showing that you're open to conversation, you're showing friendliness or you're acknowledging someone's presence out of politeness. Smiling isn't a big enough gesture to think someone likes you.
"They sent me a meme, what are they trying to say?"
A joke. People send memes as playful fun and shouldn't be taken seriously. Friends and family members send each other memes so it can't be taken as a big sign of flirting.