1) By far, 30g is not too small. There's a lot of info on smaller tanks available. While I wouldn't consider a 30g a nano tank, reading up on some of the nano tank info would be a very good idea nonetheless.
2) Tap water is not typically a good option for sw. RO (reverse osmosis), or even better RO/DI (reverse osmosis / deionization), are better options. Most LFS sell at least RO water and you could mix up your own sw, while some sell pre-mixed sw (either commercially prepared or mixed right there in the store). The reasons for tap being less than ideal are lengthy, but generally revolve around the fact that there's too many impurities that can lead to anything from nuissance algae to the death of the invertebrates in the tank. As far a salt goes, I really like Reef Crystals, but there are many brands out there.
3) Live rock will be your decoration. It will be your filter, too. Any other decoration you put in your tank will likely become covered with coralline algae and end up being less than appealling to look at. Coralline algae is good to have, btw.
4) Check out some of the fish kept in nano tanks. While 30g is above the general cut-off for nano tanks (at least in my mind), the options aren't all that different. There's so many fish available, it's hard to recommend anything without knowing what you like.
Being on a tight budget will make things a bit more difficult. Keeping a sw tank is far more costly than a fw tank. To give you a general idea, depending on what you need to buy yet, the cost for a sw tank runs from $35 - $55 per gallon, with many smaller tanks and reef tanks ending up in the mid- to upper- part of the range simply because items like live rock and sand can be bought at a substantial discount if bought in a larger quantity. The range is quite variable depending on what you plan to put in the tank in terms of fish / corals / other inverts, as there are fairly inexpensive options as well as overly expensive ones. For example, there are more common gobies than can be bought for around $10 - $20 and rarer gobies that sell for well over $100. Buying some good used equipment can help keep the costs down. Lighting, a skimmer, and powerheads can be found used relatively easily. If you're lucky, you might even find someone locally that's tearing down a tank and you would be able to save quite a bit of money on live rock, fish, and corals if the timing is right.
The higher costs not only involve money, but time. Our sw tank requires more time than the 5 fw tanks we have combined, fwiw. The smaller the sw tank, the more attention it tends to need. That doesn't mean you should go bigger, just that it could end up being very time consuming.
Good luck! And, don't worry... In keeping a sw tank, you'll learn quite a bit whether you want to or not. The more you learn before you actually get it started, the less likely you should be to make some of the more common costly newbie mistakes.
