Learn French: Je commence par un exercice facile. - I start with an easy exercise.

When talking about the first step in a process or a series of actions, French uses the phrase commencer par (to start by/with). The same goes for the end of a process: finir par (to end by/with). You generally use avec when you are accompanied by someone or using a physical tool, like eating with a fork.

Why is facile placed after exercice instead of before it?

Most adjectives in French are placed after the noun they describe. Only a specific group of common adjectives describing Beauty, Age, Number, Goodness, and Size (often remembered by the acronym BANGS) go before the noun. Since facile (easy) doesn't fall into those categories, it comes immediately after exercice.

How could I add to this sentence to say "I start with an easy exercise TO learn"?

Since you are learning how to express purpose, you can simply add pour followed by an infinitive verb at the end of the sentence! For example, Je commence par un exercice facile pour apprendre means "I start with an easy exercise to learn" or "in order to learn".

Is exercice a masculine or feminine word?

It is masculine, which is why the sentence uses the masculine article un. As a helpful trick for A1 learners, many French words ending in -ice are masculine (like un service or un caprice), though you will occasionally find exceptions.