Lots of people date coworkers. You spend a significant portion of your life at work and may naturally develop feelings for people you meet there. It can be natural to try to pursue these feelings, but the workplace is a pretty special context for meeting a significant other and that context needs to be kept in mind at all steps and phases of the relationship.

Your question was,

How do I ask co-worker out on a date?

While this question is directly answerable, it makes sense to consider the bigger picture of workplace romance. Namely, consider the following:

• Does your company have a policy about relationships? Many employers require them to be disclosed, at least, and there are sometimes very specific limits based on position or role (i.e. a supervisor may not date a subordinate, or two people on the same team may not date).
• Are you ready for this to not work out? Are you ready to keep working with this person if they say no? Or if you date for a year and then have a messy breakup? The fact that you've said you can't stop thinking about them may in fact mean that you have genuine feelings about this person, but that may also mean it will be very difficult for you to handle a rejection. You need to be ready to let this person go before you take the step of initiating a relationship.
• Do you have an "exit plan" based on that point? Before you even think about how to ask, consider how to handle no, while maintaining a professional environment.
• It's hard to give advice about feelings, but the difficult part right now is that you only know your own feelings. You don't know how your coworker will respond, or how they will handle rejection or future relationship issues. Even if you think you are mature enough to handle a "no" or a future breakup, you're taking a gamble on the other party having the same capability. Since you are the one initiating, you need to make sure your "exit plan" includes your coworker being very upset by this, to the point that they can no longer work with you. In other words, it's not fair to initiate a relationship and then expect the other party to handle things in a way that you'd be compatible with.

Also, it's worth considering that once a relationship has started, you will still be coworkers, and you will need to be able to handle that role without interruption or issue. You will also need to handle the fact that the workplace rumor mill loves to talk about workplace romances. If you two date, you need a plan for handling rumors and discussions about your relationship. If your relationship makes it past infancy, be ready to consider the following, and discuss these points with your new significant other:

• Have guidelines about how you two interact with each other at work. Make it clear between the two of you that you are employees first while you are in the office. You must still be able to get your jobs done and be productive. Sometimes, this means having strict rules: maybe you two can agree to not talk about relationship matters while on the clock. Make sure you can focus first on getting your job done.
• Have guidelines for how you act in front of coworkers. Some people are uncomfortable showing affection in any way in front of coworkers. You don't want to try try to hold hands as you walk across the parking lot on your way in to work if your significant other would be uncomfortable with that. Talking about this ahead of time clears the air and sets expectations so you won't feel rejected when they pull back from your attempt at holding hands.
• Have guidelines for what you share with coworkers or how you talk about your relationship with them. Some people are friends with lots of people at work and will naturally want to talk about things like the amazing date they went on last weekend. Other people will feel really uncomfortable if their significant other is telling others at work about their dates. Again, clear the air upfront and set expectations.

Those things out of the way, we can get back to your actual question: How do you ask out a co-worker?

Ultimately, your approach needs to be within the above framework. It's probably a good idea to begin with some work-centric interactions, or at least work-related interactions. If you have a cafe in the office for lunch or breaks, ask if you can sit with them and make small talk. It should be clear if there are feelings of interest, although you need to make sure you're not just getting excited and misinterpreting a friendly coworker for someone who is interested in you. Attraction If you feel that these casual interactions are going well, you can follow the natural progression and ask something like,

Hey, I'm having fun chatting with you, would you like to get together outside work some time? Maybe we can grab coffee or hang out this weekend.

The wording is subtle but important. While it's fine to start with work-centric interactions as a way to test the waters, when you decide to actually ask the other person out on a date, you need to make sure they understand you're asking them out on a date, and not just trying to continue workplace conversation or work-centric interactions after hours, or be "friends" if what you're after is a more romantic relationship.

The timing of when you ask can also be important - ideally, you'd ask when there isn't a strong work focus required (i.e. don't ask at the beginning of the morning standup, or during a project meeting). Asking while on a break, or at the end of the day, is appropriate because it's less likely to distract from work topics or cause a scene.

Once you've asked the question, you need to respond appropriately to the answer. In fact, it's arguable that your plan for responding to their answer is probably more important than the actual words you use to ask them out. This is especially important within the workplace context and ties back in to the first bullet list above. If the other person says no, you need to be ready to consider the following:

• Continue having professional and productive work interactions with them every day. Instantly trying to avoid them as much as possible will just make things awkward for everyone. If you can't bear to be around this person, much less keep working together after a "no," then don't ask.
• DO NOT bug them, pester them, or try to change their mind. Don't suddenly be extra nice as a way to win them over. Don't show them preference in work activities. No means no.
• Respect their privacy. Don't talk to other coworkers about how this person rejected you. Don't badmouth this person or treat them any differently. Don't give the rumor mill any fodder.