Most bandsaws comes with a guide to aid in making straight cuts.

Using the guide

However, using the guide doesn't ensure a straight cut. The blade will wander if there are any asymmetries in the blade; this happens for a few reasons -- sometimes the teeth aren't exactly symmetrical, or maybe somebody has bumped one side of the blade, slightly dulling the teeth on that side. If you have the part firmly pressed up against the face of the guide and are pushing it through the blade, and the blade starts to wander, there is nothing you can do to correct for that. And if it wanders only a little, you may not even know it is happening.

A better way to make a straight cut is to scribe a line onto your piece, and then as you're cutting, turn the piece so that the cut continues where you want it to.

Round stock can be cut on a bandsaw but some precautions should be taken. As you're cutting round stock, the blade can grab hold of the side and spin it. A simple way to avoid this problem is with a drill press vise. Lock the round piece into the vise, turn the vise over, and then make the cut while holding on to the vise.