I need some help on gerund. I have never understood any restrictions on their usage? Can soebody help. I have SC guide but there is nothing on this topic.
For example, is it correct to start a sentence with a gerund?

http://owl.english.purdue.edu/owl/resource/627/01/
that's a start. for general questions such as this one, you should probably poke around the internet for a while before consulting these forums; once you've got the general idea, feel free to post specific usage questions here.

yes, by the way, you can start a sentence with a gerund. (note the first sentence on the linked page, beginning with 'traveling').