Hi,
You can get free daily agreements only up to Taras, after that you will need to use GP's to sign agreements. GP's are unique items that you can get from pvp tournaments as rewards. If you are playing pvp also, you may be having some. Check GP's amount on your resource bar.
You need to buy articles of Argos agreement to sign it. You need to buy 4 of them each costs 25 GP's. Go to market-GP's. You can see your GP balance and the items you can buy using GP's. Buy articles of Argos and sign it.
Remember one thing, you need to sign in the order Argos-Dion-Troy-Olympia-Corinth-Pella-Sparta. So don't buy articles of the last agreement unless you signed the early ones.
I hope its clear now. Any more doubt just ask here.
With regards
Sarath