The future tense (فِعْلُ الْمُسْتَقْبَلِ) is built from the present tense by adding سَ as a prefix, or by putting سَوْفَ before the verb.
أَكْتُبُ سَوْفَ or سَأَكْتُبُ (I will write. / I'm going to write.)
See also: