I want to develop website. Using wordpress. But i've one question. If i use wordpress themes and plugins. Is there will be any issue regarding copyrights of that theme or plugins. I'll use free themes. Not premium
We can't provide legal advice here, however publicly distributed themes and plugins should be GPL or GPL compatible, so long as you don't violate the GPL you should be safe.
A good measure of wether a license is compatible with the GPL or not, is if it adds restrictions. If you can do it under the GPL, but not under the license, then that license is not GPL compatible.
As a side note, the branding of a plugin does not have to be GPL. If you buy gravity forms and resell it, you won't be violating the GPL, but you will be using their logos and marketing material ( which they will be very unhappy about, and will probably send lawyers to remind you )
Simply put in every case this depends on terms and license of specific extension.
While commonly WordPress themes (more so free ones) are distributed under open source licenses this is complicated topic.
If you need high level of confidence — you'll have to hire a lawyer with relevant expertise to check situation with specific themes and plugins you intend to use.