A pocket field guide
You do not need a name for every tree. Three small observations can turn an ordinary walk into a useful record.
Leaves
Look at a whole leaf before you look at details. Is it broad or narrow, simple or made of smaller leaflets? Then notice the edge: smooth, toothed or deeply lobed. If you make a sketch, mark the shape first and add the veins later.
Try this: collect two fallen leaves and compare their edges.
Bark
Bark can be smooth, flaky, deeply furrowed or split into plates. Young branches often look different from the trunk. Notice colour and texture, but remember that age, weather and light can alter both.
Try this: photograph the same trunk from one fixed spot in different light.
Seasons
Choose one tree close to home. Record when its buds open, when it casts the deepest shade, and when the first leaves fall. A short note every few weeks will reveal a pattern that a single visit cannot.
Try this: note the date, weather and one change each time.