If you did have some money, I would point you to the dungeon I designed for
Adventure Quarterly: Ruins Perilous, as a good example of what I would do, but as you are broke, let me offer my own thoughts.
This is advice for dungeon design, not necessarily adventure design.
First of all, I would suggest you design your first level of the dungeon to get PCs from 1st to 2nd level, the 2nd to get from 2nd to 3rd, etc. Especially with your first few dungeons. Its classic but it works.
This requires that a group have, at medium advancement, somewhere in the neighborhood of 14 level appropriate encounters. Your last encounter should be about 2 CR higher than your group, 3 CR if you want it very tough. At 1st level you want a blend of CRs ranging from 1/2 to 2. The first thing I would do then is go through and make a list of what I want my encounters to be. If you have a "theme" monster, you can use several of these, but an enjoyable dungeon is going to also include a few traps, and a variety of creatures. Pick a couple of animals, a couple of vermin, a couple of undead, etc. by going through your monster books and picking those that seem fun to you. Make sure there is a variety. Also add in a few traps. Doing this gives different classes an opportunity to "shine." (make sure to have a few locked doors and chests as well).
Now, lay out a dungeon, or a map, and do it in a way that seems logical and reasonable, and enjoyable to you. Try to have enough room(s) on the map for each of the encounters you want. As you layout, try to think of what might go where. Place your traps. Try to make them unique in some way.
Indeed, try to do something to make each encounter somehow unique. Templates are a good way of doing this, or using non-standard monsters. Once your dungeon is sketched out and you have a pretty good idea of what goes where, you stat up your monsters and write up your room descriptions. When you do each room, it helps to follow the standard Paizo format of having a general description, followed by a Creature entry, followed by a Treasure entry, followed by a Development entry.
Try to make some of the treasure as you assign it unique and interesting. Treasure is under appreciated sometimes as a way to liven up a dungeon. Try to avoid reliance on stock magical weapons. Again, aim for something unique. Give some of your treasure back-story. Make a magical weapon do something a little extra. Give it a name. There should be some coins, but also a few antiques, some jewelry, etc. Use the treasure charts to determine how much total treasure you need to provide, but put it whereever you want it, or anywhere that seems logical. Its alright for a few encounters to have no treasure and a few encounters to have lots of treasure. Dont' forget that equipment sold counts towards treasure total when doing all of this. In the end, it doesn't have to be exact but you want it fairly close. Don't worry too much about class specific items. Put in what you want and what you think appropriate and rely on your players to choose what to use and how to use it.
Pay attention to the "Development" part of your descriptions as it is what reminds you, as author and DM, what the ramifications of a room can be. If there is interaction between the creatures in different rooms, mention it here.
Don't be afraid to put in a few non-combat encounters (or potential non-combat). In the aforementioned Ruins Perilous, 1st level, I put in a tribe of "Pilfer Monkey's" that would mostly try to pick the pockets of the PCs.
One thing I try to do with each adventure I write is include 1 non-standard magical item (at least 1). This might be a unique magical weapon or a magic item of my own design. I also try to include 1 monster of my own design, or at least a monster I have pilfered and rewritten. If you do this, it will give you practice in creature and magic item design. In a dungeon I also want at least 1 trap that functions a little different than a standard trap just to keep players on their toes.
