You need to find out why it's not balancing. But it would help a lot to know if this is real life or homework. Because in real life we're generally using software to do all this, and how or why something would not balance is questionable. But if it's homework being done manually, or even real life being done manually, that is a different thing.
It's caused by math only. Work backwards. First re-add the two columns, and then figure out the difference between them. Look for the difference somewhere. Also divide that number in two and look for that. Anything on the wrong side (e.g. a credit when it should've been a debit) will cause a mistake to double itself. From there you just have to keep going backwards. i.e. the last thing done was copying balances from accounts onto the trial balance. So check all those to be sure they were copied correctly, on the correct side, in the correct amount, that none were skipped, etc. From there go back and re-calculate the balances of each account. Make sure all original journal entries balance to begin with. And if none of that works, you're stuck with the task of checking all postings made, and that is a tedious task that you want to leave as a last resort.
The real answer is that you need to be VERY careful about everything you do along the way so that it doesn't get out of balance to begin with. As tedious as that seems, it's less difficult than trying to chase down an error after the fact. And I simply cannot stress that enough.