Based on your updates in the comments, you are faced with a common situation: Company A wants to hire you and sent you an offer. Meanwhile you're still in the interviewing process with Company B which seems like a job that fits you much better.
Your first task is to get back to Company A and request some time to consider there offer. Ask them when they would need your final decision. How much leeway a company has varies enormously, but it's typically not done to ask for more than a week. If you don't accept they have to go with their second choice and they can't keep those people waiting too long.
Once you've got the deadline for Position A, go to the interview with B. If you get the idea that they like you and you don't bomb the interview, at the end of it you bring up the fact that you already have an offer elsewhere. Ask what their time-line is. If they like you a lot, well-run companies can and will fast-track you through the process. If they don't consider you a potential hire they'll probably tell you outright that they can't speed up their hiring (or reject you on the spot), but this could also be because of a rigid bureaucracy. Companies with rigid hiring policies know that they will lose good candidates but that's another topic.
With any luck, you'll be able to get an offer or rejection from B before A's deadline expires. If you don't, give B a final call the day before the deadline at A and mention that you have to accept or reject another offer and ask if they have an update. Some may get back to you that very day and offer you a job. Some wont.
If the deadline from A is there and you're still in the dark about your chances at B, it's time to make a judgement call that only you can make. Your decision to accept or reject A depends on how badly you need the job. If you're currently employed you don't lose anything if you reject A. If you're not financially secure then you might have to take the sure thing and accept A's offer. Be prepared for the fact that B might not offer you the job.