Is πλέω (I sail) an ε-contract verb? I know this is probably a really simple question, but I’m a complete beginner and I’m having trouble finding grammar help online (such as somewhere which shows the conjugation of specific verbs - could anyone reccomend?). Thanks in advance 
Not such a simple question. πλέω is an ε-contract verb but not all the time: it doesn’t contract πλέω! In forms with an ο or an ω after the ε there is normally no contraction (e.g. πλέομεν rather than πλοῦμεν) but when another ε follows contraction is normal, at least in Attic (e.g. πλεῖν rather than πλέειν). If you read a text with lots of mentions of sailing in it, e.g. Sophocles’ Philoctetes, you’ll see how it works. Later Greek seems to be more variable, but there we’re more at the mercy of the manuscripts and editors. (In Ionic πλώω comes into play and it’s even more complicated.)
This is how potentially monosyllabic -έω verbs regularly behave (e.g. νέω swim, 3 pl. νέουσι but infin. νεῖν, similarly πνέω breathe).
Smyth’s Greek Grammar is available on this site and is bound to have something on this. For specific verbs in general you should use LSJ (also on this site, but you should get your own hard-copy).
Enjoy Oxford!
Thanks for your reply, mwh! Very glad I asked this - makes much more sense now
And thank you!