It depends on the copyright usage permissions you are granted. In general for "free beats", that is ones that say they are "royalty free", you can use them provided you provide attribution (ie you say who created the beat / music wherever you send / place that beat), but it does depend on the conditions the writer of the beat / music placed on other people using it. They may place other restrictions and terms too.
It also depends on what you term "beat". if you mean the percussion only, the above generally applies, however if you mean an instrumental beat with other sounds and melodies then publishers would not consider it a new song, just new lyrics.
What you should perhaps do is contact the original writer, if in doubt, and let them know you intend using the "beat" for that purpose and ask what permissions apply. It's also nice to do things above board and let them know. If nothing else it is nice for them to know you like their beat that much.
What I wouldn't do is use something without knowing the exact permissions, in the knowledge that the owner would be happy for you to use their beat in this way. It saves later complications, and ethically it is only right that you treat fellow writers with respect.
Be aware that if you are ever regarded as a plagiarist, it will make things far harder as a writer as those who work with you will never be certain that you actually wrote anything you claim to have written.
Another option for you is to find a writing partner who can create the backdrop for your songs. Check out the collaboration board in these forums.