I don't know how far you have got in the set up process so I will have to start from the beginning. I am assuming that you have mastered basic keyboard skills such as copy and paste.
You say that you have joined Bravenet so first make sure that you are logged in to your account.
You have to select which Bravenet services you wish to use. You can always add to the list later. Register the guest book service
Once you have registered for the guest book the set up page will have a blue button at the top right labelled "Get Code". Click this and you will get options to embed or link the guestbook plus a URL for direct access. Just select the option you want and copy the code that will be displayed in the normal way.
Paste the code into your site using whatever option your site building software has for inserting code. You can't just paste it in as text. We don't know what site building software you are using, you will need to consult whatever help files come with it if you are unsure.
Posts here are from people at every level of experience. Sorry if my post seemed patronising to you but it had to cover pretty well every mistake that I have seen people posting here make.
There are people who don't realise that they have to log in.
There are people who expect to build a site without basic typing skills such as copying and pasting text.
Not realising that individual services have to be registered after opening an account is a frequent mistake.
Some people just can't see the right option for looking (including me sometimes)
And some people's posts hide the fact that they have been at this game for as long as I have.