What tank size do you have?
What equipment do you have?
What live stock (if any) do you have?
1. fishless cycling
2. biological filters - live rocks vs traditional filters (HOB
, canisters, media)
3. the fish or livestock you want to keep
There will be plenty of other things to learn, but start researching about the above and answer the question. You might want to consider freshwater if saltwater information gets overwhelming, but its not hard if you're willing to learn and research online.