I found the Inside the Volcano tour from Pinterest. But, I would highly recommend it! It was one of the coolest things i've ever done. I booked the tour through their website the day before. The tour takes you an hour and a half outside of Reykjavik to the base of the Prihnukagigur volcano.
Upon arriving you get assigned a tour guide and you make the 3k journey across the lava fields. You reach base camp and are split into 2 groups of about 6 or7 people. One group gets suited up in harnesses and helmets and go up to the volcano. The second group eats traditional homemade vegetable or lamb soup.
I was in the first group and proceeded up to the lift to down into the volcano. It takes 6 mins to go down in the lift to the bottom of the volcano magma chamber.
When we reached the bottom of the chamber we were allowed 20 minutes to look around and explore the volcano. There's guide wires to help you around the volcano because it can be slippery and the rocks could be unstable.
After we went up and had amazing vegetable and lamb soup. I definitely had 2 or 3 bowls of the soup.
While walking back to the pick up point we had some time so we explored a lava tube that was located beneath the lava field. The lava tube was really cool because you could see where the lava had hardened and flowed along the walls of the tubes.