Trust yourself!!!!!!!!!!!! It isn't that hard. That being said, there are things you will need to make it easier.
!!!!!! The thing you have to remember is that the bonding surfaces need to be extremely clean!!!!!!!!
If you are going to attempt this, here is a list of helpful tips:
1. Work in an area with plently of room to manuever around the tank.
2. With a tank that size you will need help. Get a couple of friends to hold the glass in position while you work the seam.
3. Try to work with the weather on your side. Choose a day with low humidity, no breeze, and 50-90 degrees farenheight.
4 Since you are a beginner, build your tank so that all the sides are around your bottom plate.
5. You have access to glass, so try and get the edges ground and polished. It is not a necessity but the finished product is really nice.
You will also need:
A 100% pure silicone. There are several brands out there that will work (Dow 999, GE 1000, GE 1200, GE Silicone 1) to name a few.
Paper towels
Low tack blue painter's masking tape
Plenty of disposable latex gloves
Acetone or MEK
Windex glass cleaner
Hope this helps!

Good luck!