Stitch length of 4, loosen your top tension by half a number and always pull up the bobbin threads.
If you need to gather yards of fabric then get a gathering foot. Janome make one - it looks like this
This foot is for creating soft gathers in lightweight fabrics. The underside of the foot is raised behind the needle and has a thick bar in front of the needle to gather the fabric. This works great to create the gathers as you sew. You can also gather and attach a ruffle onto a flat piece of fabric simultaneously, run the flat fabric through the top groove of the foot and the underneath fabric is gathered and stitched to the top piece, you need to keep the bottom piece feeding smoothly though so it takes practice to guide it evenly. Once mastered it is great to ruffle on valances and cushion frills.
Other option is use a ruffler foot, these will make a little ruffle every 12, 6 or 1 stitch so give a set result, and to obtain a more dramatic gather. They look like this.
The arm of the foot with a C'shape sits over the needle bar of the machine so it is moved up and down to activate the device. The little blade at the front moves backwards at regular intervals forcing a "tuck" of fabric into the feed to be stitched. Again you can ruffle and stitch to straight fabric at once but it is an art to master.
The weight of the fabric you are gathering is obviously the key variable, ginghams and poplins are lightweight so gather easily but if you need to gather something heavy like cordoroy it is going to be a battle and threads may break. If so, stitch two lines of gathers and don't try to do long runs, you'll just break a thread and have to start over.
Another good way is to zig zag over a length of perle cotton, taking care not to actually stitch the pearle at all. You can then gather up the fabric along it, this works well for heavier weights.