After casting in stitches and then knitting that row of casting stitches are always so tight. How can I fix that first row?
Welcome to KH!
You might try casting on to two needles, then withdrawing one needle before knitting the first row.
Which cast on are you using?
Could be you are just knitting tightly OR you could be twisting your stitches which makes them tighter. Check the videos at the top of the page to make sure you are knitting (or purling) the stitches correctly. If you are then you can do as salmonmac suggested or use a larger needle for casting on. If you are a new knitter make sure not to pull too tightly when knitting or casting on.