exkyo,

You need to pay your artist enough that he's willing to sign over his rights in the art to you, and you need to get that in writing (a work for hire contract).

You should get an attorney to draft the contract for you, and you need to learn the meanings of "work for hire," "copyright," and "moral rights (droit moral)", at the very least.

can i hire an attorney online to make this draft contract and use it multiple times?

Yes. There are attorney links in a sticky post atop this forum.

i dont want to pay to someone for an art, and when i start selling the game get a response from this person saying "i doesnt allow you to use my work this way, i will sue you", i want to avoid that cases.

Write a good contract. It won't guarantee that you won't get sued, but it'll greatly improve your risk.

You need to pay your artist enough that he's willing to sign over his rights in the art to you,

Since you also talked about music...

It is not necessary to find someone who is willing to sign their music rights over to you, but you do need a written agreement.

That agreement can say either

A) I (the musician) will sign my rights to the music over to you as a "Work for Hire" contract

OR

B) I (the musician) will keep my rights, but will sign a license agreement that lets you use my music in your game.

B will generally be much cheaper than A (because the musician keeps the rights to the music).

It can get more complicated of course, but those are the broad brush choices.

i dont want to pay to someone for an art, and when i start selling the game get a response from this person saying "i doesnt allow you to use my work this way, i will sue you", i want to avoid that cases.

That's the whole point of "work for hire": it isn't his work (i.e. he doesn't have copyright) because he agreed to work on your project, assigning the copyright of whatever he makes while working on your project to you. The contract needs to define and state clearly what transfer of copyright takes place to avoid the exact scenario you are already worrying about, and that's why you need a lawyer.