Did you apply for a position with this company? If not, how did they get your contact info + skillset + qualifications + etc ... ?
If they found you via legitimate means, or you applied for a position with this company, AND all of the communications so far have been informal, then as Sander Skovgaard Hansen has said ... it might not be a problem.
If this 'offer' came out of the blue, then it could be an indication that things are not what they seem. How much do you know about the company? Are they publicly traded? Do any of your trusted friends and/or colleagues know of them, or about them?
There are numerous work and Visa related scams going on all over the world, and it never hurts to be cautious.
In general, getting everything in writing is a good start ... and making sure that there are no un-written [or undocumented] conditions that are part of the package. If they're offering to sponsor your Visa, do some research and find out what issues may arise as part of that process, and find out if they expect you to pay for any [or all!] of the costs - not just the Government fees + paperwork, but any other costs, like lawyers, or even medical tests which may be required.
Perhaps I'm a paranoid old man ... but I would proceed with great caution.