check marketboards on your server. Look at what items you can gather and look at the prices/sales history (so you can see what the flux is/volume) gather things worth your time/gil. Every server's market is slightly different, but gathering the right mats+bundling them in the right sizes=decent profits.

Like for example ores tend to sell well in full stacks...but 'rocks' like granite typically sell better in smaller bunches. Again though, get in the habit of checking the MB's every day and then deciding where/what to gather based on that.