One of the Bible's most comforting truths is that God wants to guide us. He loves us, and He knows what is best for us and because of this, He wants to show us His will. The Bible says, "In all your ways acknowledge him, and he will make your paths straight" (Proverbs 3:6).
But you're right: Sometimes the devil also tries to guide us, and we have to be careful we don't follow him instead of God when we need to make a decision. How can you keep this from happening, and be sure God is guiding you? Let me suggest three steps:
First, make sure of your relationship with Jesus Christ. Have you confessed your sins to Him and asked Him to come into your life as your Lord and Savior? If you haven't done this, or if you are unsure of your salvation, turn by faith to Him and ask Him to come into your life today.
Then pray and ask God to guide you in this decision. Often, we want to tell God what we want Him to do but ask Him to help you guard against this, and to seek His will instead of your own. We must be willing to do His will, whatever it is.
In addition, search His Word, the Bible, for guidance; God never guides us to do anything that is contrary to His Word. Seek also the advice of people who can help you people who know God and have spiritual discernment. When we truly seek God's will in this way, He has promised to guide us.