Chain woes. We've all had them.
If your chain is too loose--use a smaller hook for the chain and then switch to a larger hook when you do your first row.
If your chain is too tight--use a larger hook for the chain. Your first row will thank you. Switch to the smaller hook on the first row. Less pain in your hands and wrists when you don't have to fight to get the hook in the loop.
If your chain is uneven--i.e. not every loop is the same size--run your index finger and thumb up and down the chain, tugging ever so gently. This will help to re-size the loops so that they are now more uniform in size. Repeat as many times as you need to in both directions. Do this before you start your first row.