Luke 19:45-46 (KJV)

45 And he went into the temple, and began to cast out them that sold therein, and them that bought;

46 Saying unto them, It is written, My house is the house of prayer: but ye have made it a den of thieves.

Here Jesus clears out the temple, the house of God and declares it’s actual purpose. It is a place to draw near to God, to seek Him, to pray,….
Thinking about the temple, nowadays people apply this to refer to it as the church or church building. We can conclude this from:

John 2:19-21 (KJV)

19 Jesus answered and said unto them, Destroy this temple, and in three days I will raise it up.

20 Then said the Jews, Forty and six years was this temple in building, and wilt thou rear it up in three days?

21 But he spake of the temple of his body.

Colossians 1:18 (KJV) And he is the head of the body, the church: who is the beginning, the firstborn from the dead; that in all things he might have the preeminence.
Applying the house of prayer then to Christs body the church we see that He has called us as a house of prayer.

Matthew 18:19-20 (KJV)

19 Again I say unto you, That if two of you shall agree on earth as touching any thing that they shall ask, it shall be done for them of my Father which is in heaven.

20 For where two or three are gathered together in my name, there am I in the midst of them.

(This verse relates both to prayer and church discipline)
When we gather together in His name we know He is present we can in unity pray and seek Him. It also allows also those (present in the gathering) who don’t yet know Him but are seeking to for Him to reveal Himself.
1 Corinthians 6:19 (KJV) What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own?
Here we also see that individually we are the temple of the Holy Spirit. Therefore we can consider ourselves also a house of prayer, seeking God, praying to Him just like Jesus taught us:

Matthew 6:5-15 (KJV)

5 And when thou prayest, thou shalt not be as the hypocrites are: for they love to pray standing in the synagogues and in the corners of the streets, that they may be seen of men. Verily I say unto you, They have their reward.

6 But thou, when thou prayest, enter into thy closet, and when thou hast shut thy door, pray to thy Father which is in secret; and thy Father which seeth in secret shall reward thee openly.

7 But when ye pray, use not vain repetitions, as the heathen do: for they think that they shall be heard for their much speaking.

8 Be not ye therefore like unto them: for your Father knoweth what things ye have need of, before ye ask him.

9 After this manner therefore pray ye: Our Father which art in heaven, Hallowed be thy name.

10 Thy kingdom come. Thy will be done in earth, as it is in heaven.

11 Give us this day our daily bread.

12 And forgive us our debts, as we forgive our debtors.

13 And lead us not into temptation, but deliver us from evil: For thine is the kingdom, and the power, and the glory, for ever. Amen.

14 For if ye forgive men their trespasses, your heavenly Father will also forgive you:

15 But if ye forgive not men their trespasses, neither will your Father forgive your trespasses.

1 Timothy 2:8 (KJV) I will therefore that men pray every where, lifting up holy hands, without wrath and doubting.
Let us therefore be encouraged to pray: when we are alone in our room, when we gather together for prayer,  when we gather together for worship, the reading and studying of the Word, exhortation, encouragement, communion, love feasts…. And not corrupt ourself with the ways of the world.