New guy needing help

make sure it was bought as a aquarium! as terraiums are not made as thick(since they dont hold water). filling a terrarium full with water will make it crack or leak from the seams. water weighs around 8 pounds a gallon

 

looks like you have a good start. looks like you have a display tank and a sump start for a good DIY project.

 

the bigger tank

The bigger tank looks like it was set up for reptiles? If it's had anything other than fish in it you will not be able to use it for fish without removing the silicone seams and resealing it. If you're sure there has only had fish in it then go for it. If it's been in storage it might be a good idea to fill it with water and put a cup or two of bleach in it. Make a little mark with tape where the water line is and let it sit for 3 days to make sure it's not going to leak.

 

Make sure to get your parents involved in this project immediately. Even though you have the tank, sand and rocks you could still end up easily spending close to, (or even over) $1000 dollars to get set up. If you cant afford it now, just start saving for it and plan to do it later. "Go slow and let it grow!" it's a good motto to live by and you'll probably hear it alot in this forum.