Error 500 ON EVERYTHING.

Hi, I am new to wordpress so have been downloading and uploading a few different plug ins just to get the feel for the place and play around so I know whats good and whats not.

I have come accross an Error 500 page. This is now occuring when I click on any plug in I want to activate.

I already have some that are activated but am currently testing a few different SEO plugins that allow me to add my own meta description and tags (any of you know a good one, let me know).

I have ‘All in One SEO’ which takes away my rich text options for my posts, ‘SEO Ultimate’ wont activate and gives me the error 500, ‘Platinum SEO’ wouldnt allow me to open any other pages.
There is also ‘NextGEN gallery’ which gives me an error 500.

I have looked around and some people seem to have fixed this problem by adding a line of code to their .htaccess file… but I cant seem to find mine, if I do have one. I have looked in both windows and mac… the wordpress initial download doesnt have one in the zip folder and I am lost as to what I can do to make this work as it was intended.