Hi
There's not really a way to keep track of what you've already read...and since each thread may grow and have added information each day the best way to see what's 'new' is to click 'new posts' at the top menu each time you log in.
Anything that's new since the last time you were logged in will show up for you with the thread titles bolded...
I hope that helps!
Here's
another thread about how to help keep track of threads you've started and replied to as well.