You got this, JennyEye.
My experience was similar to jfkimberly's. Go through each question carefully, mark an answer if you know it, flag if you're not sure and go back over them all, making an educated guess on the ones you were unsure on.
Even taking my sweet time, it only took 15 minutes or so. They give you plenty of time, so take it if you need it!
I took the test in 2013, so things may have changed since then, but be sure you bring the documents you need to bring (it used to be the ID you specified in the booking and proof of address). In my exam, several people were turned away for not having the right docs with them. Without the right docs, you can't even take the test (although that might have changed since 2013) so do a double check before leaving home.
Good luck!