For desktop building starters, how to build around gtx 980ti?

Alright so to start on this going to make the assumption that this is going to be a higher end PC build - assume this because you are trying to avoid bottlenecks
based on the 980Ti,

1) Its hard to get an exact number on what will bottleneck a GPU because it is game and situation dependent, also the resolution you run and settings will change what is being taxed, as a general rule I follow, low to mid range GPU should not be more then double the CPU in price (I know this is very subjective but gives a rough idea), once you get past an i5 then you are in the price range where $ to performance returns start to shrink drastically so the rule no longer really holds. So that being said for this case an i5/i7 with a 45XX, 46XX or 6XXX number should be the goal for the build

2) That is a difficult question because it depends on the duration between upgrades and how drastically things change, alot of people are running CPU's that are 4-5 years old right now and not having issues with most modern games.

3) The unlocked CPU's can have better performance if overclocked but it depends on the chip, the settings applied, cooling, etc. to get an idea of performance check this article (it uses an i5 4690k and you can see FPS results for different resolutions and some different games with overclocing and without)
http://www.tomshardware.com/reviews/build-mainstream-enthusiast-pc,3944-12.html

Mobo
1) The big things here are
a) Socket - the CPU manufacturers change the socket for their chips every few years, this means that the new socket cannot hold the old style of chips (intel just released the new 1151 socket to replace the 1150), so all future consumer intel chips (until the next socket) will be use 1151 style pins and not fit in the old 1150 motherboard
b) Chipset - using intel as the example again their Z series of chips (Z170 for 1151, Z97 for 1150) support overclocking, if you have a K series chip and a Z series mobo you can overclock, if you have just one of the two you cannot (note- some manufacturers advertise for non-K and non-Z overclocking, Intel is forcing them to remove support for this)
c) Bios and CPU support, if you chose a cheaper chipset sometimes the manufacturer will not release an updated biso to support the next generation of CPU's, Intel normally does 2 major CPU releases for each socket (it is called the tick-tock method), if you buy a motherboard in the first iteration it will need a bios update to run a chip released in the 2 iteration
2) See above regarding chipsets

So in general if you are doing a build today with a 980Ti, want it to overclock, not bottleneck and have some upgrade path you will need an
1) i5-6600k or i7-6700k
2) Z-170 motherboard (also look for SLI support so in the future if you want to improve performance you have the option of getting a 2nd 980Ti)

A good tool for this is PCPartPicker, it will help finding the best price for builds and you can share them on the forum for comments and for people to edit (very common practise on the forums)