Open your browser and use your favorite search engine. Google is the most popular and widely used for searching out topics of interest.
Type in a few key words, such as "dinosaur lessons" or "dinosaur activity preschool." If you are looking for a specific kind of dinosaur, type in the name. You will probably need to try a few different keywords. Make sure to specify "free lessons" or you will wind up getting a lot of commercial sites that require a fee for their lesson plans.
Check out the sites that your search engine lists until you find those that provide what you need. Beware of sites that require membership fees or charge for downloading. Also make sure that the site you are downloading from is a reputable site. Some places will allow you to copy and paste, others will let you print right from the online file.
Finding free material will not be difficult. Finding age appropriate lessons and worksheets will require a little more searching. Some sites, geared specifically for teachers, will offer more instructive material. Once you've collected enough ideas, you can then sit down and make out the lesson plans on how you will implement all the activities you were able to find.
Remember when searching for free stuff on the internet, some sites will require membership fees. When you encounter these sites, check around for free samples of lessons or worksheets you can use. If the material is useful, you may consider actually becoming a member if you think you'll return to that site again and again.