Note - This article is primarily intended for tournament gamers.
So you want to build a new army to play at tournaments. First you study the codex then surf the web looking for some good tips. Finally you come up with a list you think will be competitive and that you would enjoy playing. It's very important to build an army that will enjoy playing. So now you are ready to start playtesting so you can weed out the bugs and tweak your list to maximum efficiency. I rarely ever play pickup games at my FLGSs so for me playtesting is about as close as I come to casual play and that is important as well. Maybe you have designed what you believe to be a well balanced army or maybe you went for an over the top list that is a close combat monster. One thing I have learned is that no matter what I first cook up I will find some problems the first time I actually play the list. That is just the way it is, especially more so when you are playing a codex you are not familiar with and have no real prior experience. A good example for me would be something like eldar or Tau as I have almost exclusively played Space Marines or Chaos Space Marines over the years. The less familiar you are with a new army the more time it will take to debug and get just right. Therefore, it's ideal to choose people to playtest that will help you. You are not going to gain much playtesting with people who are just looking for another win and not offer you any vital feedback. I recently did some playtesting with a guy that simply told me I needed to playtest more to figure out how to build a better list - that was it as far as any advice he had to offer.
What you really need when you playtest is someone who will both give you some challenging games as well as good advice how to improve your army. Your playtesting pal should be the kind of person that will remind you about your army's special rules during a game and help you learn how to play your army. People that simply want another win to notch on their belt aren't going to help you build a better army. It's also quite useful if you can find someone you trust that plays several different armies... Sure you can figure out how to build a list that beats the snot out of Thunderwolves but how will it perform against other top tier lists such as IG or battlewagon orks? You might find it useful if you can develop a circle of trusted friends so that you can playtest with different people. This will provide you with the opportunity to gain advice from different perspectives, which is the best.
You can always take your new creation to an actual tournament to see how it fares. This will present you with the opportunity to play in competitive games and learn under actual tournament conditions. I do this from time to time but I don't go into it expecting to win anything. So if you take this approach be prepared to lose, but you will definitely have an opportunity to learn from your mistakes in an unforgiving environment. I think some people this can work well but for others they might become disappointed and possibly decide to just drop the army altogether if they don't have the level of success they originally envisioned. I've actually seen this happen too. So ideally it's always best to playtest first as much as possible before you take your new army to a tournament but eventually you will finally take your army to a competition so be prepared. You might not win much at first but you are learning and that is what is most important.