New affordable bike and accessories

Hello,
I am ready to go back to riding. I used to have a road bike I loved but it has long be gone. Now I have access to my husband's huffy mountain bike that I use once in while. I am 5'6" in fairly good shape, not so good condition anymore.
The question is what equipment do I need to buy to get back to the road? Is is possible to have a decent bike for less than $300. Any recomendations will be appreciated. I also kind of not know how to start, with the mountain bike?, at the gym? or just joining a group with pros.
Thank you for your support.

Check out your local craigslist. You should be able to get a reasonably decent bike used there. You're not going to do very well with a budget of $300 in a bike shop.