How do I form a parse tree for equations?
To produce a parse tree you need a string to parse, which you have, but
you also need a grammar to tell you how well-formed (legitimate)
strings are to be structured.

In your case you did not give a grammar. so that your question makes
no sense. Usually this is done with context-free grammars

Furthermore I suspect that what you really want is an abstract syntax
tree (AST), which is often very close to parse trees, but is supposed
to be defined more abstractedly (for example with a sorted algebra),
indpendently of any parsing process, and from which semantics is more
easily defined. Note that, while the AST may be defined independently of any arsing process, it is usually associated to textual representation via a parsing process, though other means do exist.

Below is what an AST could look like for your example. But, as for parse trees, an AST can be properly given only if you have defined formally what the abstract syntax is for your language.

To be very complete, you need at least one formal definition that relates tree structures you may be interested in (parse trees or AST) for your text with corresponding textual strutures.

              if
     ________/|\________
    /         |         \
   &gt;          if        :=
  / \    ____/|\____   /  \
 x   y  /     |     \ x    b
       &lt;     :=     :=
      / \   /  \   /  \
     y   a x    a y    b


Warning: the referenced wikipedia articles could be much improved, at least on the pedagogical side.