Before consulting an attorney, I would suggest looking through the instructions for the I-129F (fiancée visa) and I-130 (spousal visa). If you understand those and feel comfortable filling out the forms, then you may wish to proceed on your own. Any questions you have about completing the forms and submitting the accompanying evidence can be asked in these forms where many people will answer you.
On the flip side, if you don't feel comfortable filling out the forms, then legal or professional assistance may be for you.
Most people on the forum will say that if your case is straightforward, all an attorney will really do for you is complete and file the forms for you. The onus of gathering evidence will still be on you.