Are witnesses evidence? and if so, cant a person just bribe/pay a person to be a witness so he can win a case?
In some countries especially Islamic countries they are counted as evidence as they swear that it is true
A witness is not evidence, but what a witness says (their testimony) may be evidence. Or, the body of a person who happened to be a witness is evidence. I suspect that there is a translation problem. It is always physically possible to try pay a person to lie and AFAIK never legal: the person who lies and the person who induces the lie will be punished by law. The witness who testifies will have to swear that their testimony is the truth.
Are witnesses evidence?
No, witnesses testify and what they testify is evidence.
In common law jurisdictions, all evidence is either admitted because:
the parties agree to its admittance, that is, the evidence is not disputed, or
a witness testifies to it. This is how physical evidence is introduced e.g. the police officer testifies that this is the gun he found at the scene.
The trier of fact (judge or jury) decides how much weight to give to each piece of evidence - if two witnesses disagree then the trier of fact has to decide which testimony to accept (or to reject both witnesses).
cant [sic] a person just bribe/pay a person to be a witness so he can win a case?
Sure. And people can rob banks and murder each other and commit fraud. These are all things people can do. Most people don't do them because they are a) illegal and b) wrong.