Next step would be to decide what you want to grow in your garden. All of this information is under the assumption you already have a gardening area with proper sun etc. as that is a whole different thing.
On of the most important things to remember when purchasing seed is what you growing time is. That is the time between the last frost in spring and the first frost in the fall. If you don't have a general idea you can go to http://davesgarden.com/guides/freeze-frost-dates/ Now I know this is not exact as we have many micro climates that do not fit the norm in a given area. If you are doing just vegetables you don't have to worry about your zone. That is for what plants will winter over in your area.
Now the back of the seed packet or the information in the seed catalog becomes your best friend. For the purpose we are discussing here, starting seed in the house early the date to maturity is from the date the plant goes in the ground not the date you plant the seed. Other information you will get from the packet will be if the seed needs light to germinate. That means don't cover it with medium, just press lightly into the medium. There also some other guides available to you for when is the right time to start that seed inside. If it isn't your first year of starting seed you journal would be your first place to look. What did you do last year. Was your timing right? Next there is so much information available on line it will make your head spin. One book I have is Success with seed from Park Seed but you don't have to buy this book most of the information is on line at http://www.successwithseed.org/.
Also if you happen to order from Harris Seeds the have a free booklet that is very useful. It is Cultural Guide for Schedule for bedding Plant and container production. Once you have determined which variety of tomato etc. will have time to mature in your area make yourself a little chart showing when you should be starting each seed to be ready to go into the garden when it is your planting time.
I am not advocating either of these companies nor do I have any connect with them. Just where I have found information.
What plants should you consider starting early in the house. Tomatoes, peppers, egg plant, broccoli, cauliflower, cabbage, Brussel sprouts and I am sure someone can add more that I am not thinking about.