This is a short version of the song "Keys to the Kingdom"

Thy kingdom come
Let your will be done
On earth as it is in heaven

You got the keys inside this kingdom,
lift up your head, it's only begun,
keep holding on and you'll see the Son
everybody sing it now, everybody sing it now

Ooooooh ooooh oh

THY= (Old English) Your (singular)
- My lord, thy horse is ready

KINGDOM= The kingdom of God is a society of peace and justice.

COME= This is not a present tense (or it should be "thy kingdom comes", with -s), it is a subjunctive form. The subjunctive form of a verb was used to express something unreal, for example a wish:
- Thy kingdome come = I wish/hope your kingdom will come
- God save the queen = I wish God to save the queen
- God saves the queen (this is a present tense, what does God do? He saves the queen)

LET= This verb means "allow", but it is often used as an equivalent to the imperative form; the imperative form is for the 2nd person (you) and we use LET referring to the 3rd person (he/she/they):
- Open the window (I order you to open the window)
- Let Susan open the window (I'm telling you that I want Susan to open the window)
As you can see, after LET, the second verb is in infinitive without TO.
- I want TO GO // Please, let me GO
- Let your will be done = I want your will to be done, to become true.

YOUR WILL= Your wish, your desire; what you want

YOU GOT= (coll. esp. AmE) You've got, you have.

LIFT UP= Elevate, raise. To lift up your head is a metaphor that may have two meaninings: 1- look up to heaven/God (seek God's inspiration and support) or 2- be proud of yourself, be confident and self-assured.

KEEP= Continue (followed by -ing)

HOLDING ON= To hold on (in this context) is to persist, to keep trying.

THE SON= Jesus

EVERYBODY SING IT= Here "sing" is an imperative. Imperative forms are not normally used with a subject, but if we can use the subject "everybody" to emphasize that we refer to everyone:
- listen, I have something to tell you
- Everybody listen, I have something to tell you (this sounds more emphatic)
- Everybody listens (this is not an imperative, but a present tense, so we need the -S at the end)

The first part of the song belongs to the beginning of the most important Christian prayer: The Lord's Prayer. In that part, we are asking God to bring his kingdom to us, that is, a world of peace and justice.

In the second part of the song he says that we can't just ask God for peace and justice, because we have the key to that kingdom. That kingdom is God's plan for us, but it is us who must make it real. We Christians have the key to that kingdom inside us, so we must "lift up our head" (look up to heaven, pray to God) and struggle for a better world. If we persist in that, we'll see Jesus (this may either be a reference to us going to heaven because we were good, or to the Second Coming of Jesus for the complexion of that kingdom, as He promised he would)