How do I know for certain about my sexuality?

Experimenting, and not feeling guilty about it. Even if you are not gay or lesbian, there is nothing wrong with giving it a try. It's your life, no one should judge you. But makes sure you're clear about your feelings. You shouldn't lead anyone on.

Labels are not necessary, and you don't have to be certain. But, it all comes down to what feels right. If you fit a definition of a sexuality, that's probably it. The deciding factor is what you think.

you don't have to be certain your sexuality is what it is you can be certain or just make sure that you are happy with it

I struggled with grasping my sexuality when I was younger. I just naturally felt certain about my sexuality. Remember, you are the person that defines your own sexuality. There doesn't have to be a label for it.

Find out if you like the other gender? Do you have feelings for the other gender or are you attracted to the other gender?

When you've fully decided you'll just know. Are you attracted to the same gender? Are you attracted to Both genders? Could you see yourself having a long relationship with either? These are all valid questions to ask yourself, and there's no time limit to answer them.

The easiest way to find out whether you like green peas or a mango smoothie is to try them, isn't it ? :) In all seriousness, try to become more aware of your thoughts, feelings and preferences about people of both genders : try noticing your attention shifting when you are surrounded by males and females (do you tend to view one of the sexes in a more intimate / desiring way ?), notice your level of bodily arousal when/after interacting with an attractive stranger of each sex (perhaps you notice yourself smiling more involuntary or feeling slightly "shivery" or sweaty after interacting with some of them ?); ultimately (only when you are absolutely sure you are ready and comfortable with it !) you may want to try and engage in a romantic physical contact (e.g. kissing) with a consenting person of either sex and explore how you feel about it. Feeling certain about your sexuality can be a powerful, empowering, even self-defining thing. However, you do not have to define yourself if you do not feel the need or wish to do so. It is completely normal to wonder about the alternative possibilities, it is okay to give yourself as much time and space as you need to explore yourself and your sexuality.

That depends from person to person. Some people know automatically some people it takes years to know who they love but the heart wants what the heart wants. Follow it and never fear rejection.

Sexuality is not something that you can be sure of from one day to the other. It´s a process of finding out, even if that process is mostly about being comfortable with having the sexuality that you´re mostly already sure to have. The easiest way to know is to expermient. Which people do you find attraktive? Who, even, might you be able to picture yourself with? You´ll find out soon. Even sooner if you don´t overthink it ;)

In my experience, you never really know for certain. Your sexuality can change over time, and that's okay. It can be frustrating and invalidating to always change the way you label yourself, or not even have a label that you can identify with, so sometimes calling yourself queer or questioning can be a safer option.