There are no rules set in stone about making an offer on a property, after all, a house is only worth what someone wants to pay for it. All houses however do have a market value, so any offer that you do make on a property should be in line with the market value. Usually your estate agent will advise you on the offer you should make on a particular property, and take you through the offer and counter offer process to ensure that you get the best deal.
You can also do your own investigations. Look at similar houses on the same street that may be for sale. Even view them if needed, so that you can compare the price against the property that you are making an offer on. You may in some cases find that the house is over-valued, so having a better understanding of what is for sale in the same bracket in that area, will give you grounds to negotiate a better price.
Also consider other factors, such as, how long has the property been on the market? Why are the current owners selling? Are there other similar houses for sale on the same street?. By having a better understanding of the market, you may again be able to negotiate a better deal and place a lower offer. For example: the property may have been on the market for a year or more, and the owners may be selling to buy a new house that is also for sale. There may be an important deadline, meaning that the owners need a fast sale, meaning you can offer a lower price . There may be a house on the street that is in better repair, but is going for a lower price, which again would give you flexibility in your negotiations.
Most importantly, always have your budget in mind, and ALWAYS stick to this. You do not want to be overcommitted, so only offer what you can borrow from your mortgage lender. You want to enjoy your new home, and keeping a realistic eye on your finances will allow you to do just that.