Chainsets

Yes. Manufacturers have made fitting pedals relatively straight forward. Pedal threads are 9/16" 20TPI, this is the standard. There are a couple of things worth remembering fitting pedals; Firstly the left (non drive-side) pedal has a reverse thread (It screws in the opposite way to regular screw threads. Secondly, add a dab of grease onto the crank or pedal thread before screwing in. Removing un greased pedals from cranks can be very, very difficult. 

To stop your shoes touching the chainset as you pedal, adjust your shoe plates to move the shoe away from the chainset crank arm. Any changes in shoe plate position are best done in smaller incremental 'tweaks' rather than huge adjustments.

Road bike chainsets tend to come with 2 chainrings. The options of chainring sizes are usually; Compact - 34 50 chainrings, Semi compact - 36 52 and the classic race option, 39 53. More and more riders are choosing compact and semi compact chainsets which allow for smaller gear ratios for easier, faster pedalling over climbs.

Converting to 1X is a bit more complicated than just changing the chainset or chainring. For riding off-road, clutch derailleurs stop the chain from bouncing around & keep the chain taught. If you are converting a road bike, you will need to consider what to do with the front gear shift / brake lever. On a mountain bike with non integrated shift levers, it's easy to remove the left / front derailleur shifter. There are lots of youtube videos of 1X conversions, its a good idea to look into these before starting a 1X project.

This depends on how long your legs are, typically taller people have longer legs. Smaller riders would be better suited to shorter 165 - 170mm cranks, average size riders usually go for 172.5 - 175mm and larger riders usually choose 175 - 180mm. Cranks are available in 2.5mm increments from 165mm to 180mm although some sizes can be more difficult to find, especially at the shorter and longer extremes..

Yes, a smaller chainring will help. It might be more cost effective to replace your bike's cassette instead. Check your rear derailleur's maximum sprocket size. For example, changing from a 25 sprocket to a 28 biggest sprocket with a 39 chainring would give a similar gear to 34 X 25.

This depends on the groupset and the chainset. The big groupset manufacturers recommend using all their components in a groupset, however some smaller manufacturers such as Hope, Rotor and DMR make chainsets which integrate with various different groupsets.

We always recommend following manufacturers instructions for fitting any component. The Youtube channels of Park Tools and GCN Tech have very indepth and comprehensive advice for completing all sorts of jobs on bikes, these are also worth watching particularly if you are inexperienced or unsure.