I usually walk slowly through the bars and take up the offer to sit at one of the bars in the middle of the row, order a drink and sit somewhere that i can see the surrounding bars and look for where i'm going next.
You should expect the guys catcalling you as you walk around asking you to come for a drink, some will try to pull you into their bar, some will just smile and wave as you walk by. For the first time it can feel a bit overwhelming but smile and take it all in.
If you sit with a guy but decide to keep looking a small tip 100B is usually a good idea. The mamasan (manager) will usually hover around asking for a drink and/or a tip as well, if you feel like it you can buy them a drink but if they're being annoying just say not right now. It can help to have the mamasan on side, especially if you plan to be a regular at that bar and need their help to translate or find a guy that meets your needs.
Ask the guy you like to sit with you for a drink, confirm what you're looking for and ask how much they expect, pay the bill to the bar = you drink(s), his drink(s), his fee to leave the bar (off fee).
If you take a guy back to your hotel, then in the room after your services you pay his fee that you confirmed with him. Short time (ST) = he leaves after service, Long Time (LT) = usually overnight, the prices can vary from guy to guy and bar to bar so it's better to just ask him rather than assume.