When some TDs don’t know how many people will show up, they do a cop out in their TLAs and advertising, saying that “prizes will be based on entries.” That covers them for unforeseen circumstances. For the players reading the advertising, the first thing that pops to mind is, “Uh oh, they must be expecting less than 10 entries. Hmm, let’s turn the page.” Make your advertising with a definite but reasonable expectation of players who will attend. If you can guarantee prizes, do that.
What if a lot more players show up than you expect? Do several plans based on extra entries in advance. This will help you to set up new sections on game day. 100+ entrants may be extreme for your first tournament, but if you have an advanced plan for this and other “what ifs” then you will be able to react faster and not just wing it at a point where you will definitely be stressed out. Players will understand the changes if the new sections have more prizes or give them a class section where they have a better chance to win something. Nobody complains about more money being spent on them.
As has been suggested, have drinks, snacks, fruit, mints or other candy, water, coffee and tea, doughnuts and other items on hand. Decide whether you want to spend money to make these free as a courtesy to the players. Have seen this done, and it led to bigger future tournaments. Also, some organizers sell food (sandwiches, pizza, subs, specialty items) and soda at their tournaments, especially if it is at a relatively out of the way place. One guy I know used to make more money from concessions than from the tournament itself.
Do the tournament yourself and an assistant or two. Learn from your mistakes. Believe me, the players will tell you if you have done something really wrong. Create your own style for holding an event. Follow the Rulebook for doing pairings, but the rest of it is up to you. Have flyers ready for your next event. Have a sheet or slips for players to make suggestions. Stay positive, smile a lot, shake hands with players when they are leaving thanking them for coming. Cheapest form of advertising there is. Have fun.