You haven't stated in which country you are.
In some countries, law or case law sets requirements for non-compete clauses/contracts.
In France for instance, for a non-compete clause/contract to be valid, it has to:
- be limited in time, space, and scope
- include monetary compensation (it should be between one quarter and one half of the average salary)
So check the rules applicable in your own jurisdiction (if you even want to consider the non-compete).
Still, like the others, I would recommend just saying no. At this stage, unless it is really very, very well compensated, you have no reason to do so.
If however you want to agree, make sure that the non-compete includes at least the requirements above (limited in time, space, and scope, and with a monetary compensation). It should all be written down very clearly, and also define precisely what happens if the either party reneges (you start competing, or they stop paying).
When evaluating monetary compensation, weigh very carefully the scope of the non-compete contract. If it prevents you from working at all (because you're very specialised and the clause basically prevents you from working for anyone at all doing the same job, for instance), it should be equivalent to at least your salary for the whole duration of the non-compete contract, probably more as you won't necessarily be able to find a job right away when it ends, and you may have difficulty explaining why you didn't work for that duration, and of course not working also means being less "current" with your job (which for some jobs can really be a carrier-killer).
Once you factor in all that, most companies will just drop the idea of the non-compete.