How do you say i will play in french?

The English auxiliary "will" is used to indicate the future. In French you must use the future tense of the verb. For example:

She will leave at 10 o'clock. -- Elle partira à dix heures.

Forming the future tense is easy. Just start with the infinitive of the verb, then add the endings -ai, -as, -a, -ons, -ez, -ont. For example:

je chanterai -- I will sing

nous terminerons -- we will end

ils vendront -- they will sell

Some verbs have irregular future stems, but the last letter of the stem is always r. The sound of that r is the true marker of the future tense in French.