First things first... NEVER try to use the silly site finder. It's been broken for... oh at least 10 versions now. It's worthless.
I've never had a problem finding sites with no aquifer + water source... brooks are your friend, since they are like rivers with half the danger. Trees aren't all that vital to a fortress in the end... the only thing that they do for you that can't be done with metal is beds, and almost all the caravans bring at least 2 pages worth of trees on sale for dirt cheap (Traders are IDIOTS. One masterwork obsidian mug can buy out 5 pages of trees... which is probably a few hundred TONS of wood!)
As for beginning hints:
Dig in fast. Hollow out a decent sized room and set up some stockpiles to store your junk inside, cause the last thing you want are thief critters starting to steal food and critical items. Your miners will get around to anything you designate, but it may take a few minutes. Patience is your friend here.
When laying out your fortress, make all main corridors at LEAST 3 tiles wide. Trust me, when you get to triple digit dwarf numbers you'll be thanking yourself.
Remember to work in 3 dimensions. Housing a hundred dwarves can be done with only 5 zlevels of space and 4 apartment towers, and all your dwarves will be very pleased with their accommodations.
Farming is your friend: Work out how to irrigate things quickly early on. The easiest way is to dig a small tunnel to the bottom of a murky pool, stop short 1 tile, put in a floodgate, hook it up to a lever, then channel out the last tile to let the water loose. Voila, you have an irrigation system, just pull the lever to start it and again to stop it. A good precaution is to channel a 2x5 'trench' in the side of the room opposite to your floodgate to let excess water flow into... and believe me, there's ALWAYS excess water.
A very good defense is a 1 tile wide section of flooring over a 5 zlevel deep pit with traps on it. Iron serrated blades will absolutely shred about 90% of what you'll encounter early on, and make an appreciable dent in the rest of things. If the trap doesn't kill it, the fall will cripple it and leave it easy prey for even an unskilled dwarf to stab to death.
The 'Danger Room' training method will make a 10 warrior squad legendary inside of a year. It is your friend. However, my advise is to be a sexist prick and use ONLY MEN in your armed forces, as the women are too stupid to put their babies down when they enter the spiked room of infant death. Tantruming legendary axedwarves are a great source of FUN.
Cats need to be caged ASAP. Leave only two males out if possible. Alternatively, keep an eye on alerts about kitten birth and cage the little mewling bundles of bolts quickly.
The forum is your friend, and will help with most questions.