White spots on your roses could be two things - insects or fungus. To test whether it is a problem with insects or fungus, try and scrape it off with your finger. If it scrapes off it's an insect. If not, it's a fungus. If it's an insect, it's very likely scale. To get rid of it, scrub it off with warm soapy water and then apply lime sulphur (you can get lime sulphur from your local garden centre). Ideally you should apply lime sulphur in May/June when there are no leaves. If your rose has leaves, just be careful not to get the spray on them - otherwise it might burn them. An alternative to lime sulphur is conquor oil - this is not as harsh on leaves. If the white spots don't scrape off, it's a fungus. To fix this you will need a copper based fungicide (also available at your local garden centre or super market).
To prevent both insects and fungus getting at your roses, it's a good idea to spray them around spring every year with a mix of copper and conquor oil. This will kill the insects and prevent the roses getting fungus as the weather warms up. Again, try to avoid getting it on the leaves.