Psalm 1 - Blessed is the man who walks not in
Instruction for the beginning of the Christian life, which we never outgrow the need for:
How blessed to make a clearance of the past and of relationships leading away from loving the Lord first, and to delight yourself day and night in the Lord's word!

I like to recite Psalms at night upon my bed, especially if I can't sleep.

See C. H. Spurgeon's excellent commentary on this psalm.