If you have patience, i recommend starting with High Grade (HG). Any High Grade is fine, just choose the design you like. You'll also need atleast a clipper and a craft knife. As for where to buy, you can try the below:
Gundam.my
Gundamtoyshop.com
Gwinghobby.com
Gamersarena.com.my

If you want to look at the manual first before buying, you can search at hobby search english site. However the manuals are only available for most models after 2003.

If you want to build fast, you can try the  FG or AG. They are cheaper too but lacks the articulation of HG.