Question about my bike chain

[frogprincessashlie]

I have never worked on a bicycle before, but i got out my old bike to sand down and repaint. i've been taking it apart and putting it back together so i can use it while i'm working on it. but this last time i can't get the pedals to push forward. they keep locking up. i'm pretty sure i have the chain on all the right sprockets, but please let know if you have any advice or tips on what i'm doing wrong.

[cyclerUK]

Unhitch the chain from the chainset and see if the pedals turn easily.
Spin the back wheel to see if that's free.
Check the chain to see if that has seized.
With the chain back on, support the bike off the ground and turn the pedals.
Does the chain grab the chainring when trying to turn?