The ideal fermentation temperature depends on the yeast. The yeast manufacturer usually lists an optimal temperature range for each of their yeasts. For example, 18-22°C. Does your yeast specify a range? 17°C is a bit on the cold side for most ale yeasts. 19°C is a good starting temp for many ale yeasts.
The higher the fermentation temperature the faster it will ferment. But higher temps also create more fermentation byproducts and off flavors that are usually undesirable in the final beer.
Fermenting toward the lower side of her range keeps your beer tasting cleaner. When the fermentation is almost over, when the foamy krausen on top starts to collapse, you can slowly warm it up a few degrees, to help finish out and cleanup some of the byproducts (like diacetyl, the smell and taste of artificial butter).
Try to maintain the fermentation temp as constant as you can. Many of us use some form of temperature control, there's a wide variety of methods.