In brief. Try to register your trademark before you start using it (this will avoid future pb).
Decide in which country you will need your trademark as tms only offer territorial protection. If you think of commercialising your product in all europe, register a CTM (community Trade mark cost around £3-4k for 3 classes if you use and agent if you don't check the fees on UK IPO website or OHIM for CTM). Wouldn't recommend registering an international one unless you go global as very expensive.
UK tm (without agent cost or unexpected cost such as oppositions) is about £800 (with agent fees) depending as to how many classes you register.
When you chose you tm, you can either register a text eg TRADEMARK or a device eg TRADEMARK with a logo. For text you can go for black or white or colour but if you are streched penny wise go for black and white as colour only protects your TM as is (i.e with the chosen colours).
When you have chosen you TM check the classification to see in which classes you need to register your mark (i.e in respect of which products you will use it).
Then you need to draft the specifications of your TM eg: outwear children clothing but not rainwear. To be honest I would really recommend using a trade mark agent/attorney to help you, this might be slightly more expensive to start with but will avoid more costs/hurdles later one
Remember that you TM must not be descriptive and must be distinctive and you cannot use something too similar to another mark. Do a quick check on UK IPO and OHIM with as many spelling as you can and as many variations as you can of your TM to check it wouldn't be similar or identical to another one for the same products. Would also try a google search (worldwide).
That is it in a nutshell, you are probably now quite confused and thinking "blimey". I am not even going to try to explain the registration process with IPO otherwise this post will be very long indeed. I would really advise taking a TM attorney as it would help greatly. Protecting your Intellectual Property and getting is right from the start is v v important as the value of your business will rely on it.
Goog luck
www.itma.org.uk/intro/index.htm
www.ipo.gov.uk/
oami.europa.eu/