3 things combine to make your hero a strong one. Its level, its gear level and its star level. As you level up your hero, every 10 levels you can equip a new set of gear. Each 10 levels allows another 15 pieces of gear to be placed on your hero with a combined total of 15 points.
In total you gear can add 150 points (15 per every 10 levels up to a total of 10, lv100).
Getting you gear up should be one of the most important things you should do at the start of the game along side farming medals and getting promoted. The materials needed for gear can be one in an abundance on the wheel even if it does take a huge amount to get your gear to the max. You can also farm the materials. Alongside the materials you need a script for every time you want to upgrade a piece, scripts can also be farmed.
You should never ever upgrade your stars before you have your gear at Lv10. The reason being for every level your gear goes up your stars go down. And because the bonus of the stars is very low at low levels it can be an awful waste of gems to upgrade them just to down grade them. An informed player will be able to get their gear full within a couple of weeks if they use the farming methods listed on this site.
Buying the materials can be a waste, like i said the wheel and advanced farming can get you everything you need for full gear and the fact that if you bought all the materials they would cost around 440 dollars means it pays to put in the time and farm them yourself.