First of all, welcome to WGT!
To be honest, you will want to stay at the bottom tier for as long as possible. Consider it your chance to start learning the game and its mechanics.
You won't need to upgrade your gear yet, just use the starter gear and work on lowering your score. For equipment, make SURE you do not use the EZ swing clubs. They are horrific.
The best way to earn credits without spending money is doing offers, watching videos, and completing surveys. You can search the forum for more information on obtaining free credits.
For training tips and advice, check out the game tips section in the forum. It has tons of tutorials and helpful advice. You can also search youtube and find some great videos on the WGT.
Hope this helps a little!