Keep attempting to transfer an ever decreasing amount until you succesfully make a transfer and pay the fees when asked.
That is normal.  If you cannot send the amount that you typed in, just lower the amount until you can send successfully.

So now you have the wallet you mined to and a newly created wallet (like a savings account) that you transferred coins from your mining wallet.

You can now or copy  the mining wallet to somewhere safe.

You now create a NEW mining wallet.  But I advise you as a newbie to not use P2Pool.  I used https://www2.coinmine.pl/prt/index.php.  I am very pleased with this pool
because you can set your automatic withdrawl amount to saw blocks of 500 PRT ( or another amount ).