When you first start homeschooling, it can be very overwhelming! There are so many questions and decisions to make.
You can buy a pre-packaged or "boxed" curriculum. That means everything you need for an entire school year for a certain grade level comes in a box.
Or you can put together your own curriculum by buying a certain book for each subject.
Sometimes boxed curriculum is pretty expensive. Personally, I prefer to put my own together because I think it's fun and it can really save you money. But it does take time.
First you can look at something called a scope and sequence (can also be called a typical course of study). It is a list that tells you everything your child needs to know for a certain grade level. Here are two.
http://www.educationworld.com/standards ... ndex.shtml
Then it is a good idea to read reviews of curriculum before you buy anything. Here are some links where you can do that.
As far as knowing whether or not your children are on grade level..........do you know if they were taking any achievement tests while in school? Do you have the results from those tests? Or end of grade tests? If they were doing well on those, then they are probably okay. Here is a link that has a few placements tests online that you could use to evaluate if needed.
Since you are just starting out, I would recommend that you only teach the basics -- math, english, history and science, as you said. Then you could add the other 'extras' later when you feel a little more comfortable. Take it slow so you don't get discouraged.
For math, I highly recommend a curriculum called Teaching Textbooks. (It does all the teaching for you!) We have been using it for years and we love it! It comes with solutions cds so when your child misses a question, they can always find out why. Here is the link. (It is expensive, but worth it)
Teaching Textbooks was on the list of placement tests (above) so if you choose to use it, your children could take the test and you would know if they are on the right grade level.
My favorite place to order curriculum from is a company called Rainbow Resource. They have a huge selection and low prices. They also have great reviews.
There are lots of ways to save money when buying curriculum. You can use your public library. I like to check out books for science and history especially. (then you don't have to buy a curriculum for those subjects - it is free!) Also, there are so many free resources online! For example, if your child needed to study the Civil War, you could google "Civil War for kids" or "civil war lesson plans" and find so many useful sites. You can also search for used curriculum to buy online. Here are some links for that.
Hope this helps some. Best wishes!