First off, I recommend posting this hello in our group for Internal Communities. There's several thousand community members just like you ready to welcome you into the fold. If you want to move this post, you can select the Move option under the Actions menu in the right sidebar. Then type in "Internal Communities" and select it to make the move.
My recommendations for a good start are to go through the free training Community Manager Training for either users or Community Managers.
Then I recommend reading through the posts in the Internal Communities to get a feel for what other people are doing? You could also search for terms like "intranet replacement" and things like that. There's also a great post that I started myself before I worked for Jive Corporate communications and your communities.
Welcome to the community and feel free to let me know if you have any other questions!