1. I want to check out.
我要结账。
还能这样说：
I'm going to pay.
I want to settle the bill.
应用解析：
check out 还有别的意思：检查;通过能力测试;试试，看看。
例如：Would you check out your change?
请检查一下找的钱好吗?
2. How can I pay?
我要如何付钱?
还能这样说：
What way of payment can I choose?
How shall I give it to you?
应用解析：
pay up 付清全部欠款;
pay-as-you-earn 预扣所得税法;
pay sth. out 花费;放松;
pay for sth. 为某事吃苦头或受惩罚;
pay sth. back 还给某人
3. Can I buy it on installments?
我可以分期付款吗?
还能这样说：
May I pay by installments?
Can you sell it to me on an installment basis?
应用解析：
installment on sth. (分期付款的)一期付款。
例如：keep up the installments on the house 为买该房按时分期付款
4. Will that be cash or charge?
您是用现金付账还是用信用卡?
还能这样说：
Would you like to pay by cash or by credit card?
How are you going to pay, in cash or by credit card?