I'm afraid I disagree with Lonestar a little, I'd forget about the weldon #16. For something this small, 1/4" will work fine, but try to get cyro FF, which is extruded, but it has pretty good welding properties. If you want to make it stronger you can get cast acrylic, but I wouldn't get anything other than cyro GP or spartech polycast. Cheap cast is worse for welding than cyro FF.

You don't glue acrylic, you actually weld the joints with solvent, that's the weldon #3. Might be better to use #4, it's a little slower acting and the joints might come out a little better. All you do is cut your pieces so the edges are really clean and everything fits together perfectly. Usually building a box you'd weld the sides to the ends, then the whole thing to the bottom. You have to take care and make sure the edges of the sides/ends line up just right so the whole thing will fit to the bottom well. It's not that hard, it just takes some patience. There's a monster thread on reefcentral, something 100 pages long, about acrylic fabrication, with lots of pictures and advice. You might want to read some of that. You get the weldon #4 (or #3) at the acrylic place; regal is fine. They'll even cut your pieces for you and finish the edges on a router which makes everything fit better, but you'll have to pay them for that.

If, when you finish, there are leaks or lots of bubbles in the seams, a last resort fix is to shoot a bead of weldon#16, which is basically the solvent in #3 thickened with acrylic resin, along the seam. It never looks good if you ask me, and it's not very strong. This is a small tank so probably it won't matter. Much better than weldon #16 is weldon #40, which is a 2 part polimerizing cement for acrylic and will really fix any leaks. It's hard to find locally, though, and it really stinks when you're mixing and applying it.

Oh yeah, you need a needle/squeeze bottle applicator to apply the solvent; you just line up your joint and run the needle along the seam. The solvent will wick into the joint, melt the acrylic at the joint, then evaporate quickly, which makes the acrylic dry into a nice watertight joint. It just won't fill any gaps at all. When I'm lining up joints I use a slip of paper as a feeler guage to check for gaps. If I can get the paper in the joint anywhere, I redo the edge until there are no gaps.

Have fun!