Not everyone likes you. That can be hard to accept, especially if you're a generally nice person who makes an effort to get along with people.
Sometimes you just have a personality clash with someone at work. Maybe there's no real reason you don't get along, while on other occasions there's a real reason for the dispute. It may not be your fault: Maybe you both have the same ambitions, or the other person has decided that you're an obstacle. The reason doesn't really matter, but there are some clear things you should do when you don't get along with a coworker.
Be the bigger person
Even when you genuinely don't like someone because they have chosen not to like you or to treat you poorly, it's important to act professionally. That means you need to be cordial and involve the other person where appropriate. You don't have to eat lunch with the other person (unless it's a group situation that makes that appropriate), but you do have to act as if nothing is wrong.
Aside from being professional, handling this situation depends upon the level of dispute, and that's where your judgment kicks in. Are you just rivals who want the same promotion, or is there some true bone of contention between you? If there is, can that be solved by talking things out, or should human resources (HR) be involved?
If the situation seems like a misunderstanding, it's OK to schedule time with the person to try to talk it out. Doing this may not make you the best of friends (or even resolve the dispute) but it might relieve the tension. There's a difference between being two people who just don't connect well and two people who don't like each other.
Sometimes, just making sure it's not personal can make working together easier. In other cases, you may learn that there's a reason for the dispute, and you're actually at fault (maybe inadvertently).
It's only necessary to loop HR in if the dispute causes real problems in the workplace. That has to go beyond not liking someone or them not liking you. Involve HR if you feel in danger, or like the dispute has caused you to be excluded from work or discriminated against in the workplace.
You can't solve everything
It's OK to have some people not like you. If it's just that, then you have to let it go. That can be hard, and it might be a bit uncomfortable in a small office, but sometimes people just rub each other the wrong way, and it's unavoidable.
If that's the case, focus on being above the fray. In many ways, you should pretend that nothing is wrong. Say hello in the morning, and try to make your work interactions as professional as possible.
Over time, that may cause the dispute to become less important, or maybe even to melt away. If not, acting professionally can make the office more comfortable. It can also demonstrate true leadership. If you can work with someone who clearly doesn't like you, then you probably can work through other tough situations, and that's a valuable skill to possess.