Well, aiming depends on aiming and stroke depends on a lot of things. First and foremost if it looks like you are hitting centre cue ball, and it looks and feels like you are cueing straight it will most likely be a sighting issue. Having the vision centre off slightly can make you apply anywhere upto a tip and a half of unintended side spin. Check out the 'Ask The Instructor' sub-forum. There is a thread on there about sighting correctly in which I detail and provide a link to a more in depth thread I started on the matter.
Without seeing you play I would then have to assume your alignment isn't quite right. If the elbow and grip aren't both in sync with each other and on the same line...the line of aim...then it adds a slight 'arc' either left to right or right to left into your stroke. Other possible causes can include gripping too lightly or too firm, movement in the head, having a wrist that is too wobbly or having the grip too far back or forward. That's a lot to look out for, so it is easier to just stroke as slow as possible and really pay attention to what the tip is doing. Slow the back swing right down, make a conscious effort of focusing on the tip, pull it back nice and slow and make sure it comes back perfectly straight. Then pause slightly and switch your focus to the OB whilst trying to keep as still as possible then accelerate through the cue ball. If that cures the problem then its your backswing. You aren't pulling the cue back straight. The forward swing is a direct result of what you did on the back swing. Pull back straight and you will cue straight.
Don't have the cue ball following or drawing back from the OB when trying to figure this out. Just hit medium range stop shots firm with the measles ball and this will tell you what your stroke is doing. After you master the stop shots then work on trying to follow the cue ball into a pocket with no side spin.i have a little stroke test of my own I do when I play snooker. Blue on the spot, white in baulk dead straight into the corner and force follow the white into the pocket with the blue. Its a tough tough test of your stroke. My record is 9 in a row. If you can do this 9 times in a pool table you have nothing to worry about with regards to your stroke.