Even tension comes with practice.
Do you mean gauge, as in stitches per inch?
Knit up a swatch with the needles and yarn called for in the pattern that's about 5" x 5 ". Then measure how many stitches you have over four inches.
If you have more stitches than the gauge of the pattern, then you need to go up a needle size. If you have too few stitches, you need to go down a needle size.