Sub-directory Site

I have a website that has the latest version of wordpress installed with a particular theme.

I have also created a sub-directory for my blog and installed wordpress (no theme installed for this though at this moment in time).

I want my blog to have the same theme and navigation menu as my main site, but i am struggling with this. I am not sure if i need to do this multi-site stuff or something else.

All i want to do is create separate sub directory pages for each separate page on my site, but link them all together with the same navigation bar so that people appear to be on the same wordpress site, even though they are on a sub directory.

I am not sure if this all makes sense. Any help on this will be really appreciated.