the key (to success) is (doing something)

An idea, a person, a technique, or anything that's very important for making something successful is called "the key":

My key to success has always been getting out there and practicing harder than anyone else.

Dalton's hard work was really the key to the success of the entire event.

Publishing so much content is a constant struggle. The key is not to spend too much time on any one part of it.

There are some other phrases that use the word "key" in this way:

The key factor in my decision was the cost. The Panasonic was a hundred dollars cheaper.

Senator Donnely was a key player in getting the bill passed.

establish a relationship with (someone/some group)

There are different kinds of relationships. Some examples are:

• romantic relationships
• relationships with your family members
• business relationships

When you're talking about starting a business relationship, you use the word "establish":

We're hoping to establish an ongoing relationship with a manufacturer in Ireland.

It's a little strange to use "establish" to talk about starting other kinds of relationships. Use "start" instead:

I'm sorry. I'm just not ready to start a relationship.

A business relationship can be formal, meaning that there's been a specific agreement for two companies to work together. Or it can be informal, which means that companies do business together because it seems like a good idea at the time.

You can also establish a personal relationship with a business contact. That might include getting to know things about their family, their hobbies, and their tastes.

a client

"Clients" are similar to "customers". But the word "customer" suggests someone buying things at a store or directly on the Internet. The word "client" is used in business-to-business sales:

I have a meeting with a client this afternoon.

People have long meetings with their clients, take them out to lunch, and try to sell them products or services for large amounts of money. When a client buys something, they usually use their company's money, not their own personal money.

The word "client" can mean the person that you do business with, or it can also mean the company that they belong to:

Microcorp is a client of ours.