SSL usage

Hi, as a new user, just want to confirm the use of SSL that we are about to purchase:

I want to make sure that only certain pages on our site are encrypted, for obvious performance reasons.

So, my understanding is, that if an 'href' is used, we substitute http with https to use the certificate, correct?

However, how do we use the SSL in our code, with response/redirect to a relative path in our web site programming? I assume, anyhow, that relative path always means, the initial url remains in force.
What would be the performance impact if the fully qualified path, i.e. with http or https is used in our redirects?

Thanks and regards
gandlu