If the trademark you use is already listed in the Trademarks Register, check whether the goods and services for which that trademark was registered are the same kind as those for which you use or want to use your trademark. If they are, it may be the case that your use of the trademark is infringing someone else's trademark rights. The best way to avoid a dispute is to stop using the trademark for the goods and services in question as soon as possible. In such cases, it is best to devise a new trademark that does not encroach on anyone else's territory.
If the goods and services for which you want to register the trademark are not the same as those covered by the trademark you found in the register, i.e. are of a different nature, in many cases you can simply proceed with your trademark application. This is, however, not the case when the pre-existing trademark is a well-known one.
If you are unsure whether your trademark can be registered, consult a specialist.