How can you find out what type of plant is in your garden?

There isn't enough of information to answer this question.

Is it an evergreen tree with needles or deciduos tree, that looses its leaves in the Winter? If it is deciduous, then: Does it have simple or compound leaves? Does it have lobes (like maple or oak trees) or teeth (like birch or beech trees)? What size it is? Is it one of the common shade trees planted along the street or ornamental trees that usually blooms in Spring? Does it flower? When? What color? Does it get fruit or seeds? What color does it turn in the Fall? What kind of bark it has? Anything would help....

It also depends if the tree is growing in Florida or Alaska, because different trees grow in different climates.

Get a tree identification guide from the library or show it to someone who knows trees (even a leaf might help).