you don't have to have a hydrometer right away. i would recommend getting one in the next few weeks as funds become available. also get a theif (about $9 at your LHBS)
1. don't follow your kits instructions with regards to time. It will probably say let sit for a week and then bottle. let it sit for about 3 weeks (this is why you have plenty of time to get a hydrometer).
2. relax, don't worry, have a home brew. (RDWHAHB) you'll make mistakes. it happens.
3. your brew day won't be perfect. you might accidently do any of the following: leave out an ingredient, drop something into the wort, stick your finger (or arm) into the wort, pitch your yeast too hot, have a boilover, forget to aerate your wort, forget to mix the wort and water, forget to sanitize your spoon, take too long to cool your wort, and a host of other things. if any of these happen, see #2. its harder to mess up beer than you think. don't make it a habit of making these mistakes, but you and your beer will live.
4. sanitize, sanitize, sanitize
5. sanitize, sanitize, sanitize