This page will help you if you can't remember the difference between the Present Continuous tense ("I am doing"), and the Present Simple tense ("I do").
Which tense you use depends on how you see the state or action. If you use the Present Simple ("I do"), you think something is permanent. This means we often use the Present Simple to talk about general and scientific facts, our routines or habits, to give definitions and to describe things.
The Present Continuous ("I am doing") means the action is happening now (or around now), is unfinished, or temporary. We use it to talk about trends and changes, or about situations happening now that are different from normal.
"I live in London." (This is my home.)
"I am living with my parents." (A temporary situation until I buy my own house.)
"Hot air rises." (A scientific fact.)
"House prices are rising." (A trend happening now.)
"I drive to work every day." (My routine or habit.)
"I am walking to work this week." (My car is being repaired.)
"At work I write letters to customers." (My job routine.)
"I'm writing a difficult letter." (This is what I am doing right now and I haven't finished yet.)
So the most important rule is that you use the Present Simple ("I do") for permanent states and the Present Continuous ("I am doing") for changes and trends.
Some verbs cannot be used in the Present Continuous tense. This is because they already suggest permanence.
Verbs of possession - own, have, belong. (Although you can say "I am having a party", it doesn't mean you own the party, it means you are hosting a party.)
Verbs of the mind - believe, know, think, forget.
Verbs of emotion - love, hate, detest.
Verbs of the senses - see, hear, smell, taste. ("I'm seeing the doctor tomorrow" means I am going to visit the doctor, but "I see the doctor" means "the doctor is standing in front of me".)
Verbs of geographical location - lie. ("London lies on the River Thames".)