I use www.beercalculus.com
, as my friend has beersmith. The calculations are comparable and good to start with. If you really like making your own recipes buy beersmith.
There are a few ways I start with making my recipes. If you want to make a specific style then you will have to do some research to decide if what you have fits that style. OR you can take a recipe that is already made and modify it. This helps to ensure you have a good base and you can taste what your modification did easily.
And lastly, you can just take your ingredients and throw them all together. I've done this a few times and all but one have turned out great. This is not a preferred method, just something I do when I notice I've got a few straggling ingredients lying around that need to be used.
I am still a novice at this, but if you post your ingredients and what type of beer you like you will get some good recipe recommendations from others on the forum. They are all very helpful with this type of thing.