Antistatic straps/mats are nice to have but definitely not a must-have. I've assembled pretty much all of my PCs without them, and you also don't need a full set of screwdrivers. One decent phillips screwdriver is enough 99% of the time, and for grounding you just touch a heater or other grounded piece of metal.
For PSU wattage, I love the calculator that is linked in my signature
Fill in your hardware, let it calculate a wattage, round up to the next 50W step and add another 50W on top, tada, good estimate of what PSU you need.
General rule of thumb for tightening screws, especially on coolers if you applied thermal paste yourself: tighten them in a cross shape step by step.
When you have 4 screws in a square shape
1 2
3 4
you tighten 1 a bit, then 4, and then 2-3 or 3-2. Rinse and repeat until all of them are tight.
The most finicky part of PC assembly is usually getting the mobo (with CPU, cooler and RAM already mounted) into the case. Check the position of the mounting "bolts" just in case, some cases allow different mobo sizes and some of the bolts have to be repositioned for that, but for ATX and micro-ATX that's normally not needed. But checking just in case doesn't hurt, i'm just a bit careful on that part since I fried my first mobo due to a bolt in the wrong spot...
Push the ATX bezel (or however you guys call that thing, did I mention English isn't my main language?
) into place first (it should snap into its hole in the case), then get the mobo in, align all the ports with the bezel and push it in place so all screw holes align with the bolts. put the screws in loosely until all holes are filled and tighten them then.