Most Verizon phones ship unlocked, so chances are you don't need to. But you do need to check if the phone you have will support whatever frequencies Straight Talk uses or you might find yourself with only voice service, no 4G data, or nothing at all.
Easiest way to check is to ask someone on a different carrier to let you try their SIM card in your phone. If it works, your phone is already unlocked.
Verizon model of Galaxy S5, G900V, will work on Straight Talk's 2G band but will only work on their 3G CDMA 850 & 1900 MHz bands and on their 4G/LTE Band4-AWS. As SpookDroid suggests, try a Straight Talk SIM card in your phone first.
If you want to use the phone on Verizon towers it has to be paid off. You would have tobuy a Verizon Straight talk bring your own phone kit. You can buy the full kit with all the sims at a Walmart for $60. Then you have to call customer service to find out if Verizon will approve the phone on their towers. Like someone said before most Verizon phones that are sold in last couple of years a carrier unlocked. I you want to use the phone on Tmobile or ATT towers just activate the sim on the carrier you want. The put the sim in the phone and change the APN to the which ever carrier you choose to use.