I have worked (present perfect) and I worked (past simple 1)

When we talk about something that happened in the past, but we don't specify precisely when it happened (perhaps we don't know, or it is not important to say when it happened), we use the present perfect.

We have visited Thailand before. 

I have complained about the traffic before.

When we use the present perfect, it suggests some kind of connection between what happened in the past and the present time. Often we are interested in the way that something that happened in the past affects the situation that exists now: 

I've washed my hands so that I can help you with the cooking. 

We can't go ahead with the meeting, because very few staff have attended. 

The connection with the present may also be that something happened recently, with a consequence for the present:

I have exercised for a year. I look fit now. 

My ceiling has fallen in and the kitchen is flooded. Come quickly!

When we talk about how long an existing situation has lasted, even if we don't give a precise length of time, we use the present perfect:

The city has changed a lot since we last visited it. 

Prices have fallen sharply over the past six months.

We have recently started to walk to work instead of taking the bus.

We often use the present perfect to say that an action or event has been repeated a number of times up to now: 

They have been to Thailand four times. 

I have traveled around this city five times. 

When we want to indicate that something happened at a specific time in the past, we use the past simple. We can either say when it happened, using a time adverb or assume that the hearer already knows when it happened or can understand this from the context:

The train departed two hours ago. 

I met him an hour ago. 

I received a newspaper at 8 AM this morning. 

We use the past simple for situations that existed for a period of time in the past, but not now:

When I was younger I ate vegetables. 

The Pharaohs ruled Egypt for thousands of years.

If we are interested in when a present situation began rather than how long it has been going on for, we use the past simple. 

Compare:

I had this injury two months ago. (PAST SIMPLE) 

I've had this injury for two months. (PRESENT PERFECT)

When did you arrive in Britain? (PAST SIMPLE) 

How long have you been in Britain? (PRESENT PERFECT)

However, we also use the past simple to talk about how long something went on for if the action or event is no longer going on:

I stayed with my cousins for seven months. (= I am no longer staying  with them)

I studied in London when I was 20 years old. 

How long did he live there?