Since you are just starting out on phone repairs. Avoid doing iPhones with bent frames for now as there will be issues mounting the screen on properly and the board could have other issues due to flex of the logic board.
This is trivial but you may want to also avoid doing iPhone 5s battery replacements and up as they are hard to take off the battery but it can be done if you can pull the battery adhesive tabs off without the tab breaking.
Changing the volume button / power button cable on iPhone 5 and 5s requires the battery to be taken out.
Also avoid doing power button or home button only repairs on iPhone 4 / 4s until you are familiar and confident in separating screens to replace the home button as breaking the screen can always be a possibility.
With power button repairs it is possible to fix them without taking out the screen but you can risk damaging the screen cable (as taking out the two screws for the power button is close to the lcd / digitizer cable, so better not to do that until you have more experience on it.
For teardowns I recommend buying a dead iPhone 6 that you can buy cheap so you can practice tearing it down.
When you take off the front camera assemblies that come with the proximity cable on it (iPhone 5 and up) you need to be very careful prying them. I start from prying from below the proximity sensor to get the glue loose for the proximity sensor plastic gasket which is attached to the screen frame / glass with the same glue I mentioned.
Anyway the repair guides on here help a lot so better to read through them too if you haven't already.