Your question is pretty vague. Perhaps you should find a Realtor to work with if you don't have one already.
In general, you have to decide what you are will to take for your house, then go from there. Will the market sustain your price or is the low ball over unjustified? Do you want to only negotiate the money or other terms (such as close of escrow time, as is sale) on the contract? Do you play hardball or one step at at time? All these can play into how you do a counter offer.
Perhaps you should engage a Realtor's service. Negotiation is a huge part of the process and then follow up with other negotiation for repairs and credits after the house goes into contact as well as making sure both sides fulfill the contract terms in a timely matter are all part of the Realtor's services.
A Realtor with great negotiation skills is worth their weight in gold.
More important there are always terms and conditions to an offer to consider aside from price. If there are terms or conditions of the offer you do not understand your best bet would be to retain a local real estate attorney to advise you on the offer/contract before making a counter offer in writing.