In this country it is really quite easy. We have such a limited flora compared to most places. Learn 10 tress and you will know more than 98% of people, really, I have tried this everyone thinks they know a few trees so you ask them which they can identify, the list often dries up at 4 or 5 and rarely passes 10. So out you go with a book of a camera and then back to check on the web.
Always look for seeds or flowering parts first, an acorn, an alder cone, catkins are much more distinctive than leaves which can be variable. Then look at leaves and the way they are spaced on the twigs, opposite pairs or alternate? last look at bark but be aware it changes a lot depending on the age and size of a tree so sweet chestnut for instance has very smooth bark up to about 10" diameter but an old one is very gnarled and often has spiral bark.
Learn the easy 10 first, lets say choose from these birch, hawthorn, oak, beech, ash, larch, scotts pine, rowan, alder, hazel, elm, lime (quite a few of these are split into several different species but don't wory about that to begin) depending on where you are you could have others like cherry, hornbeam, whitebeam, poplars.
If you learn 20 you are definitely into the top 1% of tree identifiers in the country...a sad state of affairs but true.