Psalm 20
"May the L-rd answer you on the day of trouble" — said in Shacharit and customarily recited in hard times and for healing.
Psalm 121
"I lift my eyes to the mountains — from where will my help come?" A psalm of trust and protection, customarily said for safekeeping.
For livelihood
The Talmud (Berachot 4b) teaches that whoever says Tehillah LeDavid (Psalm 145) three times daily is assured a share in the World to Come; it contains "You open Your hand and satisfy every living thing." This article is for learning, not a halachic ruling; for practical questions, ask a qualified rabbi.
Sources
Tehillim Chapter 20 · Tehillim Chapter 121 · Tehillim for healing · Psalms for parnasa