Thank you for asking such an important question. The basic qualifications for a church worthy of your membership are given to us in Acts 2:42 - "They were continually devoting themselves to the apostles' teaching and to fellowship, to the breaking of bread and to prayer."
In this verse there are 4 characteristics that must be present if it is a "good" church:
1) "continually devoting themselves to the apostles' teaching" - in our day, this emphasis finds its expression through solid teaching and preaching of the Bible. Here we are not talking about using the Bible as a spring-board for social commentary, but a solid devotion to Scripture seen in the verse-by-verse expository preaching. A church that is worth joining will "preach the Word."
2) "continually devoting themselves to . . . fellowship" - This characteristic doesn't mean that they have alot of activities or dinners (although these can be useful), but that people have a desire to share in what Jesus Christ is doing in their lives. True biblical fellowship is coming together and sharing in the joy of what God is doing in your lives - cp. 1 John 1:3-4;
3) "continually devoting themselves to . . . the breaking of bread" - this is a phrased used of dining together in Scriptures. However, there is a special aspect of breaking of bread established by the Lord on the night He was betrayed - often called the "Last Supper" or the "Table of Communion." Here we see a reference to the regular observance of "Communion." In observing communion, we are told that it is to be a time when individuals "examine themselves" (1 Corinthians 11:28) and deal with the issues of personal sin prior to partaking. Hence, I believe a church that is worthy of membership is a church that regularly provides a challenge to its people to deal with sin through a time of self-examination and communion around the "Lord's Table" or "Communion."
4) "continually devoting themselves to . . . prayer" - a clear reference to a church that believes in the power of God and our dependency on Him for all things. A church that does not hold prayer up as a priority is not worthy of one's membership.
I believe that all four of these characteristics ought to exist in a church that is a biblically based ministry. Many churches have slick packaging of programs and entertaining services but fail to challenge people with the constant emphasis on Scripture. If the church you are looking at meets these four criteria, condifently involve yourself and serve Christ fully.