You did not specify a grade level, but as a teacher mentor, I can help you. The first place you may want to look at is at your State Education Website. Most of the states now have a teacher portal that contains lessons and activities matching their state standards. Some districts have a similar portal as well.
There are a number of "free" sites on the web. These sites have lessons designed by classroom teachers. Some of the commercial sites where you pay a subscription fee or pay for lessons do not always have classroom teachers designing the lesson. As you know, designing a lesson for students if you do not classroom experience is difficult. I am listing a few of the non-profit or .org sites. Commercial sites with lesson plans are easily found in a web search.
The National Education Association (NEA) has great lessons for most age groups and ability levels. Organizations like Teacher.org are good sources as well. PBS Learning Media and Discover is an excellent source for lessons. They have a wide range of activities that coordinate with various media on the sites.
I have added a few more links on the bottom of the page you may want to look at. For example, Commons Open Educational Resources (OER Commons) and Read, Write, Think, and Read Works are excellent resources for reading and language arts materials. Core Knowledge is another source you may want to check out.
My advice when looking for lesson plans and activities is simple. First, ask a colleague if you are a school teacher. If you homeschool, then look to your state first for guidance and your local homeschool association. Second, contact your state and local departments of education. Third, look for sites with ".org" that have teachers creating the lesson plans. There are some really good commercial sites available, and I don't want to discourage you from looking at them. The ".org" sites tend to be sponsored by education grants or government funding and offer a broader choice of state-aligned lesson plans. Finally, one often overlooked source for materials are state and federal parks. Most of them have education components and maintain a supply of free materials related to the site.
Commons Open Educational Resources: https://www.oercommons.org/
Core Knowledge: https://www.coreknowledge.org/community/teacher-workroom/teacher-created-lesson-plans/
Read, Write, Think: http://www.readwritethink.org/
Read Works: https://www.readworks.org/