I would like to add to @SwankyLegg's precise answer that in order to select a place for your signup form you may want to think about your conversion goals.
Signup forms are part of a marketing strategy that have the objective to extend your communication channels, in this case, extend the channel into your visitors' inbox.
Regarding the above statement, then think about your signup form as another call-to-action element in your site.
If we think about UX, the signup forms are, as @asfollows said in his/her comment, not what users are looking for initially and is not until they discover the value-proposition of your product, that they'd probably choose to enter their signup information.
There are good tools to know if your site visitors are engaged with your proposition in a first visit, this is by knowing one or several of the following:
- How far have they scrolled
- Where is the visitor's cursor at an specific height of the whole page
- How long has the visitor been actively in the site
If you manage to know one or more of the above, then it should be easy to know where to place any of your calls-to-action.