There are two kinds of tense: past and present.
Chosing one tense and sticking to it is very important. I hate finding tense-switches in books, especially published books. It irritates me to no end.
Past tense is the most common, and reads like this:
Teague stopped, glancing behind him. Was he being followed? He didn’t see anyone, but…that didn’t mean much.
Present tense reads like this:
Gripping the stair rail, Raine creeps down the steps, freezing as one creaks under her weight.
Mixing tenses feels cheap.
While past tense is more common, present tense is gaining popularity, and gives an immediete, sometimes more intense, feel.
Most of my stories have been in past tense, but one short story and my latest novel, Hosanna House are in present. The switch has been a little bit odd (and messes me up when I go back to edit a past-tense), but I’m enjoying it a lot. For some reason it just seemed to “fit” the idea for Hosanna House–it started out that way in my head, and when I tried to go traditional, it simply didn’t work. Whichever fits your particular story, you must stay consistent.