Our pastor spoke on these two verses a couple of Sundays ago and it was quite amazing all you can get out of them.
Verse 12 talks about how we need to work out what God has given us. It isn't saying we need to work for salvation, lets get that straight right away. God graciously gives us the gift of salvation through Jesus, we don't need to do anything to earn it. When someone gives you a birthday or Christmas gift it isn't because you earned it, it is because they love you. Anyways, moving on...
How do we "work out" our salvation? We have to live Christ filled lives that shine His glory. Sin kind of hinders us from doing that though. Humans are really weak. We can't do much on our own. That is why there is verse 13.
WE DON'T DO IT ON OUR OWN! God works through us! After salvation there is sanctification. Sanctification is God changing us to be more like Jesus. It doesn't happen in an instant, if it did Christians would be perfect and wouldn't need Jesus. No, we need Jesus because we are weak, sinful humans. We can't do it, but God can. God can make us love that unlovable person, God can make the right answer come out of your mouth in a disagreement, God can make us say no to something that we shouldn't do. If we go on our own will everything will be a mess. That is why we work out what God works in us. God doesn't do everything for you though. You can't just lay in bed and wait for God to do something. You need to get up and put effort in so He has something to work with.
He says to work it out with "fear and trembling". One day we will all bow down at the feet of Jesus. We will confess He is Lord. What are we to even have Him as King? We are NOTHING compared to Him. Thanks be to Him who died for our sin so that we can stand before Him spotless.
Finally, God takes pleasure in us doing good. God actually takes pleasure in us?! It is God's joy, happiness, delight, amusement when we are more like Jesus. That should make us strive to be more like Jesus when we know that it makes God smile.
Verse 12 can't be wihtout verse 13 and verse 13 can't be without verse 12.
I hope that was somewhat clear. As our old pastor always said, "Was that clear as mud?"
God bless y'all,