"First thing I have a question about is the weighing of pellets...why do you do this? Do you try and group them by weight before shooting? Do they differ that much out of the same tin that this is neccesary?"
Weighing of pellets...Yes it can be a benefit. Many pellets list their weight on the can/box. One can also find the weight of a particular brand on the net. MANY brands/types of pellets don't weigh their advertised weight. This IS important if you have a chronograph and are taking velocity readings and calculating muzzle energy figures as well as energy figures down range. EXAMPLE: I have a Beeman Kodiak .25. It shoots a Diana dome pellet at 817fps. RWS/Diana ADVERTISES this pellet to weigh 21gr but in fact mine averaged 19.78gr. If I used the advertised 21gr weight to calculate ME figures it would be 31.13ftlb. But in reality it is a 29.32ftlb gun. Also, MANY field target shooters seperate pellets by weight. That's a little too serious for me!
"What is a seater? and where/when do you use it? and for what kind of guns?"
Don't waste your money. Mine STAYS in my shooting box!
"Another question is where do you get an airgun cleaning kit? or do you even clean the barrel after shooting?"
Another "don't waste your money." I wipe off each airgun after shooting with Shooters Choice "Rust Prevent," Beeman MP-5, or Break-Free CLP. This is VERY important as your sweaty hands are all over an air rifle. I have an R-1 about 20 years old, rust free. I rarely clean the bore of an air rifle. If I put one up for storage for a lengthly period I'll run one of the above products thru with a "Patch Worm" or an Otis kit, either will be all you will ever need. The only other reasons to clean the bore on an air rifle is if accuracy deteriorates OR you're shooting at velocities over 1000fps where barrel leading can be a problem. I usually wait for the accuracy to go south.
"Also, using a chronograph(?) do you need to have one of thse? What kind? are they expensive?"
Currently, I have four different bands of chronographs. I consider a chronograph almost a "must buy." It's hard to tell what you air rifle is doing without one. I recommend a "Pro Chrono" as a best buy. I have Oehler, PACT, Chrony, AND Pro Chrono and for the money, the Pro Chrono is the one. They're about $100.00.