You will normally want to choose 2 Hymns and a piece to walk in to and another to walk out to ('Processional Music').

Processional pieces can usually be any well know classical piece.  The most commonly chosen pieces are:

Wagner's Bridal Chorus and the Wedding March by Mendelssohn

For hymns, the website oremus.org is particularly useful and will play you samples of the tunes.

(Just enter the hymn title or a line you remember into the search box there.)

Popular hymns include:

And did those feet (Jerusalem)
Be thou my vision
Come down, O love divine
Dear Lord and Father of mankind
Give me joy in my heart
God, in the planning and purpose of life
God of all living (to the tune: Morning has broken)
Guide me O thou great Redeemer (Bread of Heaven)
Immortal, invisible, God only wise
Lord of all hopefulness
Love divine, all loves excelling
Make me a channel of your peace
Morning has broken
O Jesus I have promised
O perfect love (tune: Finlandia)
O praise ye the Lord
O worship the King all glorious above
Praise, my soul, the King of heaven
Tell out, my soul
The King of love my shepherd is
The Lord’s my shepherd
To God be the glory