[SalesForce] How to update a record in a trigger with value from a REST API

When an account is saved I need to update it with value from a REST API. I though my approach would work but I'm getting the following compilation errors.

Method does not exist or incorrect signature: void getTicker() from the type tickerClass

What's the proper way to make a REST API callout in a trigger to update the trigger records?

I tried getting it to work like this, but got stuck on the compilation error.

1. Person clicks New Account, and fills in the appropriate fields. Person clicks Save.
2. triggerTicker on Account is triggered.
3. triggerTicker calls tickerClass.
4. tickerClass uses REST API to grab the 'high' value.
5. tickerClass adds the 'high' value in the valueHigh variable and then assigns valueHigh to the Ticker_Start__c field.
6. Record is saved.

My trigger.

trigger triggerTicker on Account (before insert) {

    tickerClass.getTicker();

    // convert the output of ticker.getTicker() from String to Decimal
    Decimal newTicker = Decimal.valueOf(tickerClass.getTicker());

    // create new Account record and include required fields
    for (Account newAcc : Trigger.new) {
        Account a = new Account();
        a.Name = 'Ticker Test';
        a.Ticker_Start__c = newTicker; 
    }
}

It calls a class (tickerClass) method getTicker(). I am getting the following Problem in the on triggerTicker LINE 'Decimal newTicker = Decimal.valueOf(ticker.getTicker());:

Here is tickerClass.getTicker():

public class tickerClass {

    @future(callout=true) 
    public static void getTicker() {

        String retVal = null;

        Http http = new Http();
        HttpRequest request = new HttpRequest();

        request.setEndpoint('https://www.bitstamp.net/api/ticker/');
        request.setMethod('GET');
        HttpResponse response = http.send(request);

        if (response.getStatusCode() == 200) {
            Map<String, Object> results = (Map<String, Object>) JSON.deserializeUntyped(response.getBody());
            system.debug(results);

            retVal = string.valueof(results.get('high'));
            Decimal valueHigh = decimal.valueOf(retVal);
        }
    }
}

UPDATE: UPDATED THE TRIGGER AND THE CLASS BUT STILL GETTING ERRORS. I AM HAVING ISSUES WITH JUST PASSING THE ACCOUNT FROM THE TRIGGER TO THE CLASS. IDEAS? CAN'T FIND ANYTHING IN TRAILHEAD.

Trigger (updated)

trigger triggerTicker on Account (after insert) {
    tickerClass.getTicker(Trigger.new);
}

Class (updated)

public class tickerClass {

    // have to call future method if you are going to pull the method in a trigger
    @future(callout=true) 

    // because we are looking for the accountId we need to make the trigger an after trigger
    public static void getTicker(Account[] accounts) {

        // do callout stuff
        Http http = new Http();
        HttpRequest request = new HttpRequest();
        request.setEndpoint('https://www.bitstamp.net/api/ticker/');
        request.setMethod('GET');
        HttpResponse response = http.send(request);

        if (response.getStatusCode() == 200) {
            Map<String, Object> results = (Map<String, Object>) JSON.deserializeUntyped(response.getBody());
            String retVal = null;
            retVal = string.valueof(results.get('high'));
            Decimal valueHigh = decimal.valueOf(retVal);

                for (Account acct : accounts) {
                    acct.Ticker_Start__c = valueHigh;
                }
        }
        // save the changes
        update accounts;
    }
}

tickerClass Error
'public static void getTicker(Account[] accounts) {'
Future methods do not support parameter type of List
Ok, I'm thinking I need to pass over the Account Id, right? How do I do that?

triggerTicker Error
'tickerClass.getTicker(Trigger.new);'
Method does not exist or incorrect signature: void getTicker(List) from the type tickerClass
I get one of these errors no matter what I do it seems. I have no idea how to properly pass data to the Class it seems…