I noticed that you posted this info at MetalMeet, which is a good place to search for this info.
Anyway, to answer some of your questions:
What guage sheet? 3003 AL around .063 (16 gauge I believe)
How do I weld? Two choices, gas or arc welding (TIG). Welding AL can be tricky. It is very easy to blow through because you have to use a lot of heat and move quickly...not to mention trying to keep track of the puddle.
How do I ensure that there is no leak? When the tank is complete, pressure test it by capping all openings (petcocks, fuel filler) and injecting low pressure into tank will submerging tank in water. Existence of bubbles indicates pinhole leaks.
Mounting points? Those wil have to be fabricated to fit the mounting on your frame.
The first thing you need to learn is how to metalshape. You need some simple tools to do that. The most basic tools you need are a hammer, sandbag and a hollowed out wood stump.
Go to the MetalMeet site and do a search for 'getting started with metalshaping' or something similar.
To do a tank like the ones indicated, you need a buck to reference against. Do a search on MetalMeet for 'making bucks' and 'flexible patterns'.
What you're attempting to do isn't easy, but it can be done.
I speak for experience because I have made a gas tank for a motorcycle, and am currently making another one for a Cafe style bike.