At work contemplating bigger things.

Trying to figure out salavation.. It is a free gift.. but how is one saved and how do you know for sure? My knowledge of the bible is weak at best.. who is correct? call on the name of the Lord and be saved? what does that really mean? What if I sin... and fall..? What if you only believe in Jesus was the substitutionary atonement for your sins but do not get the rest of the bible..