First things first. Your problem here is that you have vertical dropouts, and it is quite difficult to get proper chain tension because you can't adjust the wheel axle position. You can mess around with different chainring/cog combinations, a chain half link, an eccentric axle hub, and eccentric bottom bracket and even filing the dropouts, but this is both complicated and expensive. Personally, I'd just try to find another frame with either track dropouts (rear facing) or horizontal dropouts (forward facing). Maybe you can either sell or trade your current frame with the vertical dropouts.

As to the subject of gear inches, it is a number computed as follows: (number of crank chainring teeth) X 27 / (number of hub cog teeth), and is just a number that is used to determine gearing. A higher number means a higher (harder) gear and a lower number means a lower (easier) gear. I'd recommend a starting gearing of about 70 GI, such as 44 X 17. Remember that a smaller rear cog equates to a higher gear, so if you went from a 15 to a 14 you'd end up with an even higher gear than you have now.