could you post some sort of a SEO manual for invixion to.. or will the phpbb manual of yours also help me there??
No and no.
i specially need to learn about creating meta tags. do they really help?
To a very limited degree, yes. See here:
Note that the title tag is very very important for seo.
What would happen if I directly edit the profile_add_body.tpl?
Just tell me whether it would screw things up or not
It depends on what you do. I can certainly envision you screwing things up that way.