How would you answer the following questions?
Do you renounce the spiritual forces of wickedness,
reject the evil powers of this world,
and repent of your sin?
Do you accept the freedom and power God gives you
to resist evil, injustice, and oppression
in whatever forms they present themselves?
Do you confess Jesus Christ as your Savior,
put your whole trust in his grace,
and promise to serve him as your Lord,
in union with the Church which Christ has opened
to people of all ages, nations, and races?
How about the following two?
As members of Christ's universal Church,
will you be loyal to Christ through The United Methodist Church,
and do all in your power to strengthen its ministries?
As members of this congregation,
will you faithfully participate in its ministries
by your prayers, your presence,
your gifts, your service
and your witness?
If you can answer "I do" to the first three, and "I will" to the last two, and have been baptized or are willing to be Baptized. (If you've been Baptized before, we won't re-baptize you, so long as you were Baptized in the name of the triune God), then you can be a member of a United Methodist Congregation. These come from "The Baptismal Covenant", which is our official liturgy regarding reception into the United Methodist Church. When a child is Baptized, those questions are asked of that child's Parents or Guardians. Baptized children are not considered 'professing members', but are 'Baptized Members'. They're Baptized into the church with the promise that they'll be raised in and grow in their Christian faith. When they're older, they'll go through 'confirmation classes', growing deeper in their faith, and will become 'professing members', and those very same questions that were once asked of their parents will be asked of them. In your case, as an adult, they'll be asked directly of you.
SOME Churches will require new membership classes or some sort of education prior to those questions being asked. But ultimately, those are the questions being asked.
Membership is solely the responsibility and authority of the appointed Senior Pastor in the United Methodist Church, there is no congregation wide vote; for example. So largely, meeting with the Pastor is how you'll determine exactly what you need to do. Doctrinally, however, that's all you need to 'affirm'.
Where your beliefs go beyond those basic, foundational questions determine, perhaps, the level to which you can serve the church. I once had someone share a desire to teach Confirmation, but whose beliefs staunchly differed with the United Methodist Church, being opposed to the Episcopacy (Bishops), the appointment system, viewed sacraments as 'symbols' among other things. They were more than welcome to be a member of the church. Unfortunately, teaching Confirmation would be off the table. As my colleague,
@circuitrider pointed out, the qualifications for becoming a Pastor in the UMC are far stricter and while even THEY don't require complete and total affirmation of the Book of Discipline (though must comply with it, but we don't prohibit clergy from stating a belief that they disagree with part of it. Within reason), there's a great deal more continuity with the UMC that's required there.
I'm curious about your statement with the sacraments, could you say more? The United Methodist Church affirms a "Real Presence" doctrine, and affirms Holy Communion to be a Sacrament. Unlike our Baptist Brothers and Sisters, and some other Evangelical and Charismatic churches, who view Communion as an Ordinance or a Symbol. We are, essentially, Anglican/Episcopal in our view of Holy Communion.
A closing thought: Why do you want to be a Member? Being a non-member doesn't preclude you from any part of the life of the church. In fact, you can even serve on certain committees as a non-member. If you're unsure about membership, don't sweat it. Continue to worship, participate in the life of the church, et al. You don't need to be a member in the UMC. In fact, though culturally it's changed, Methodism has historically been a denomination where only the congregations leaders and most faithful were members. There is no expectation that everyone who attends regularly become members.