You can create a hashtag simply by appending the hash symbol to a word, like this: #hashtag. #socialmedia,#conferences

How to Utilize Existing Hashtags:

There are a wide variety of already established hashtags — and new ones being created daily — that you can join. Some examples are #musicmonday,#tastytuesday, #followfriday.

How to Find a Hashtag:

Some great site to search hashtags being used are:

What the Trend?: This useful little service makes it really easy to learn about trending hashtags. When something starts trending, What the Trend? will provide a quick blurb on what’s going on.

Twubs: Twubs  uses a wiki system to help disseminate information on a hashtag. It aggregates tweets and imports pictures to help illuminate the topics being discussed.

Hashtags.org: While not the best at helping you understand the meaning behind a tag, Hashtags.org is good at showing you its use over time and recent tweets, which oftentimes is enough to figure out the meaning behind the tag.

Tagalus: Tagalus is a simple dictionary for hashtags. It’s very easy to find information on thousands of hashtags as defined by other users. You can also define a hashtag by tweeting tagalus.

How to Start your own Hashtag

The first step in creating a hashtag is deciding on the tag word itself. You should pick something memorable, easy to spell, and perhaps more importantly, as short as possible. Remember that Twitter gives everyone just 140 characters per tweet, so no one wants half of it to be taken up by an unwieldy hashtag. Once you’ve figured out the tag itself, the next step is simple: start using it and promoting it. Make sure your tweets using the hashtag are worthwhile and add something of value to the conversation.

More resources for hahstag information:

HOW TO: Get the Most Out of Twitter #Hashtags

The Twitter Hash Tag: What Is It and How Do You Use It?

Ultimate Guide to Twitter Hashtags | Search Engine Journal