I am a newer believer and sometimes get frustrated over all of the different doctrines in various churches. Can you help me sort these out?
You bring up some issues that have been debated for centuries and will be debated until the Lord comes.
The main thing in trying to solve these problems is to not be side-tracked from the main issue, but to grow in your knowledge and love of the Lord Jesus. Satan likes to distract us from our focus on Christ (who alone is worthy) and to cause division and confusion within the body (the Church), but God is not the author of confusion. The apostle Paul addresses these issues in his prayer for the saints at Ephesus, that God would give them a spirit of wisdom and revelation of the knowledge of Him and that the eyes of their hearts may be enlightened to behold the majesty, glory and power of our Lord. (Ephesians 1:15-23). Jesus said the Holy Spirit would guide you in all truth (John 16:13).
We have the Word of God and while it is true there are many things about which people debate, the essential things are clear. It is important not to accept ideas as being true if they do not agree with God's word. Remember the Bereans when they heard new teaching, they checked it out with the Scriptures to see if it was so (Act. 1:11). When we read Scripture we need to ask the Holy Spirit to enlighten us and to guide us in all truth.
It isn't possible to address all your questions in this letter. However, I would recommend getting hold of a great resource book, Systematic Theology by Wayne Grudem, (Zondervan, 1994). A more concise version (and a little less expensive) is Bible Doctrine by Wayne Grudem.
The most important thing is to keep our eyes on Jesus. 1 Peter 1:3-11 should help you understand one of the questions you raised about assurance of salvation. Of course there are many other scriptures too.
Concerning tongues (one of the gifts of the Spirit—and perhaps the most controversial one), remember, the Holy Spirit gives each one of us gifts to edify the body, i.e. the church. But if we seek the gift rather than the giver we are not exalting the Lord but focusing on ourselves and this is very wrong.
People who hold various opinions usually have favorite isolated scriptures that support their views. Some seem contradictory and can be confusing if we allow them to be. Paul addresses this problem in I Timothy 1:3-7.
I hope this has been of some help to you. I encourage you to keep things in proper perspective and keep focused on Christ and grow in your faith. In time the Lord Himself will reveal these things to you as you read the Bible and pray.