In 1 John 1:3, God calls us to have fellowship both with Him and with other believers:

"That you also may have fellowship with us; and truly our fellowship is with the Father and with His Son Jesus Christ."

Fellowship with god:

There is an oneness, a bonding in the knowledge of Jesus Christ, He dwells in us (Ephesians 1:3-4) and He is our companion. We have fellowship with Him (1 Corinthians 1:9) for all eternity. Through the blood of Jesus Christ, we have been reconciled to God.

God cannot dwell where there is sin, and since we all sin, Jesus came to pay the price for our sin. When we believe in Jesus, What He did for us allows us to be reconciled to God. God told us in 2 Chronicles 7:14: "If My people who are called by My name will humble themselves, and pray and seek My face and turn from their wicked ways, then I will hear from heaven, and will forgive their since and heal their land."

We are not to have fellowship with workers of darkness:

Ephesians 5:11-12: "And have no fellowship with the unfruitful works of darkness, but rather expose them. For it is shameful even to speak of those things which are done by them in secret."

Fellowship with other believers:

One of the essential building blocks of spiritual growth is fellowship with other believers. Often the best way to do this is to become part of a local Church. This provides you with spiritual instruction from God's word, allows you to worship God with other believers, enables you to use your God-given gifts and abilities as God intended, and makes you accountable to spiritual leadership. You draw strength from other believers and in turn, you lift them up. Some people may think they do not need this, but think about it - if joining in fellowship with other believers was not important, why did Jesus establish the Church? (Matthew 16:18).

Fellowshipping is something you "get to" do, not something you "got to" do.